feat: Add AuxTxBuilder (cosmos#10455)

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## Description

Closes: cosmos#10443 

For creating an intermediary/auxiliary tx (e.g. by the tipper in tipped transactions), using the existing `client.TxBuilder` is awkward. We propose a new client-side builder for this purpose.

API Usage (e.g. how the tipper would programmtically use this):
```go
// Note: there's no need to use clientCtx.TxConfig anymore!

bldr := clienttx.NewAuxTxBuilder()
err := bldr.SetMsgs(msgs...)
bldr.SetAddress("cosmos1...")
bldr.SetMemo(...)
bldr.SetTip(...)
bldr.SetPubKey(...)
err := bldr.SetSignMode(...) // DIRECT_AUX or AMINO, or else error
// ... other setters are available

// Get the bytes to sign.
signBz, err := bldr.GetSignBytes()

// Sign the bz using your favorite method.
sig, err := privKey.sign(signBz)

// Set the signature
bldr.SetSig(sig)

// Get the final auxSignerData to be sent to the fee payer
auxSignerData, err:= bldr.GetAuxSignerData()
```

auxSignerData is a protobuf message, whose JSON reprensentation looks like:

```json
{
  "address": "cosmos1...",
  "mode": "SIGN_MODE_{DIRECT_AUX,LEGACY_AMINO_JSON}",
  "sign_doc": {
    "body_bytes": "{base64 bytes}",
    "public_key": {
      "@type": "cosmos.sepc256k1.PubKey",
      "key": "{base64 bytes}"
    },
    "chain_id": "...",
    "account_number": 24,
    "sequence": 42,
    "tip": {
      "amount": [{ "denom": "uregen", "amount": 1000 }],
      "tipper": "cosmos1..."
    }
  },
  "sig": "{base64 bytes}"
}
```

Then the fee payer would use the TxBuilder to construct the final TX, with a new helper method: `AddAuxSignerData`:

```go
// get auxSignerData from AuxTxBuilder
auxSignerData := ...

txBuilder := txConfig.NewTxBuilder()
err := txBuilder.AddAuxSignerData(auxSignerData)

// Set fee payer data
txBuilder.SetFee()
txBuilder.SetGasLimit()
txBuilder.SetFeePayer()

sigs, err := txBuilder.GetSignaturesV2()
auxSig := sigs[0] // the aux signer's signature

// Set all signer infos (1st round of calling SetSignatures)
txBuilder.SetSignatures(
  auxSig,                   // The aux SignatureV2
  signing.SignatureV2{...}, // The feePayer's SignatureV2
)

// Sign
signBz, err = encCfg.TxConfig.SignModeHandler().GetSignBytes(...)
feepayerSig, err := feepayerPriv.Sign(signBz)

// Set all signatures (2nd round of calling SetSignatures)
txBuilder.SetSignatures(
  auxSig,                   // The aux SignatureV2
  signing.SignatureV2{feepayerSig, ...}, // The feePayer's SignatureV2
)
```

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

client/tx/aux_builder.go

@@ -0,0 +1,212 @@
+package tx
+
+import (
+	"github.com/gogo/protobuf/proto"
+
+	codectypes "github.com/cosmos/cosmos-sdk/codec/types"
+	cryptotypes "github.com/cosmos/cosmos-sdk/crypto/types"
+	sdk "github.com/cosmos/cosmos-sdk/types"
+	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
+	"github.com/cosmos/cosmos-sdk/types/tx"
+	"github.com/cosmos/cosmos-sdk/types/tx/signing"
+	"github.com/cosmos/cosmos-sdk/x/auth/migrations/legacytx"
+)
+
+// AuxTxBuilder is a client-side builder for creating an AuxSignerData.
+type AuxTxBuilder struct {
+	// msgs is used to store the sdk.Msgs that are added to the
+	// TxBuilder. It's also added inside body.Messages, because:
+	// - b.msgs is used for constructing the AMINO sign bz,
+	// - b.body is used for constructing the DIRECT_AUX sign bz.
+	msgs          []sdk.Msg
+	body          *tx.TxBody
+	auxSignerData *tx.AuxSignerData
+}
+
+// NewAuxTxBuilder creates a new client-side builder for constructing an
+// AuxSignerData.
+func NewAuxTxBuilder() AuxTxBuilder {
+	return AuxTxBuilder{}
+}
+
+// SetAddress sets the aux signer's bech32 address.
+func (b *AuxTxBuilder) SetAddress(addr string) {
+	b.checkEmptyFields()
+
+	b.auxSignerData.Address = addr
+}
+
+// SetMemo sets a memo in the tx.
+func (b *AuxTxBuilder) SetMemo(memo string) {
+	b.checkEmptyFields()
+
+	b.body.Memo = memo
+}
+
+// SetTimeoutHeight sets a timeout height in the tx.
+func (b *AuxTxBuilder) SetTimeoutHeight(height uint64) {
+	b.checkEmptyFields()
+
+	b.body.TimeoutHeight = height
+}
+
+// SetMsgs sets an array of Msgs in the tx.
+func (b *AuxTxBuilder) SetMsgs(msgs ...sdk.Msg) error {
+	anys := make([]*codectypes.Any, len(msgs))
+	for i, msg := range msgs {
+		var err error
+		anys[i], err = codectypes.NewAnyWithValue(msg)
+		if err != nil {
+			return err
+		}
+	}
+
+	b.checkEmptyFields()
+
+	b.msgs = msgs
+	b.body.Messages = anys
+
+	return nil
+}
+
+// SetAccountNumber sets the aux signer's account number in the AuxSignerData.
+func (b *AuxTxBuilder) SetAccountNumber(accNum uint64) {
+	b.checkEmptyFields()
+
+	b.auxSignerData.SignDoc.AccountNumber = accNum
+}
+
+// SetChainID sets the chain id in the AuxSignerData.
+func (b *AuxTxBuilder) SetChainID(chainID string) {
+	b.checkEmptyFields()
+
+	b.auxSignerData.SignDoc.ChainId = chainID
+}
+
+// SetSequence sets the aux signer's sequence in the AuxSignerData.
+func (b *AuxTxBuilder) SetSequence(accSeq uint64) {
+	b.checkEmptyFields()
+
+	b.auxSignerData.SignDoc.Sequence = accSeq
+}
+
+// SetPubKey sets the aux signer's pubkey in the AuxSignerData.
+func (b *AuxTxBuilder) SetPubKey(pk cryptotypes.PubKey) error {
+	any, err := codectypes.NewAnyWithValue(pk)
+	if err != nil {
+		return err
+	}
+
+	b.checkEmptyFields()
+
+	b.auxSignerData.SignDoc.PublicKey = any
+
+	return nil
+}
+
+// SetSignMode sets the aux signer's sign mode. Allowed sign modes are
+// DIRECT_AUX and LEGACY_AMINO_JSON.
+func (b *AuxTxBuilder) SetSignMode(mode signing.SignMode) error {
+	switch mode {
+	case signing.SignMode_SIGN_MODE_DIRECT_AUX, signing.SignMode_SIGN_MODE_LEGACY_AMINO_JSON:
+	default:
+		return sdkerrors.ErrInvalidRequest.Wrapf("AuxTxBuilder can only sign with %s or %s", signing.SignMode_SIGN_MODE_DIRECT_AUX, signing.SignMode_SIGN_MODE_LEGACY_AMINO_JSON)
+	}
+
+	b.auxSignerData.Mode = mode
+
+	return nil
+}
+
+// SetTip sets an optional tip.
+func (b *AuxTxBuilder) SetTip(tip *tx.Tip) {
+	b.checkEmptyFields()
+
+	b.auxSignerData.SignDoc.Tip = tip
+}
+
+// SetSignature sets the aux signer's signature.
+func (b *AuxTxBuilder) SetSignature(sig []byte) {
+	b.checkEmptyFields()
+
+	b.auxSignerData.Sig = sig
+}
+
+// GetSignBytes returns the builder's sign bytes.
+func (b *AuxTxBuilder) GetSignBytes() ([]byte, error) {
+	auxTx := b.auxSignerData
+	if auxTx == nil {
+		return nil, sdkerrors.ErrLogic.Wrap("aux tx is nil, call setters on AuxTxBuilder first")
+	}
+
+	body := b.body
+	if body == nil {
+		return nil, sdkerrors.ErrLogic.Wrap("tx body is nil, call setters on AuxTxBuilder first")
+	}
+
+	sd := auxTx.SignDoc
+	if sd == nil {
+		return nil, sdkerrors.ErrLogic.Wrap("sign doc is nil, call setters on AuxTxBuilder first")
+	}
+
+	bodyBz, err := proto.Marshal(body)
+	if err != nil {
+		return nil, err
+	}
+
+	sd.BodyBytes = bodyBz
+
+	if err := b.auxSignerData.SignDoc.ValidateBasic(); err != nil {
+		return nil, err
+	}
+
+	var signBz []byte
+	switch b.auxSignerData.Mode {
+	case signing.SignMode_SIGN_MODE_DIRECT_AUX:
+		{
+			signBz, err = proto.Marshal(b.auxSignerData.SignDoc)
+			if err != nil {
+				return nil, err
+			}
+		}
+	case signing.SignMode_SIGN_MODE_LEGACY_AMINO_JSON:
+		{
+			signBz = legacytx.StdSignBytes(
+				b.auxSignerData.SignDoc.ChainId, b.auxSignerData.SignDoc.AccountNumber,
+				b.auxSignerData.SignDoc.Sequence, b.body.TimeoutHeight,
+				// Aux signer never signs over fee.
+				// For LEGACY_AMINO_JSON, we use the convention to sign
+				// over empty fees.
+				// ref: https://github.com/cosmos/cosmos-sdk/pull/10348
+				legacytx.StdFee{},
+				b.msgs, b.body.Memo, b.auxSignerData.SignDoc.Tip,
+			)
+		}
+	default:
+		return nil, sdkerrors.ErrInvalidRequest.Wrapf("got unknown sign mode %s", b.auxSignerData.Mode)
+	}
+
+	return signBz, nil
+}
+
+// GetAuxSignerData returns the builder's AuxSignerData.
+func (b *AuxTxBuilder) GetAuxSignerData() (tx.AuxSignerData, error) {
+	if err := b.auxSignerData.ValidateBasic(); err != nil {
+		return tx.AuxSignerData{}, err
+	}
+
+	return *b.auxSignerData, nil
+}
+
+func (b *AuxTxBuilder) checkEmptyFields() {
+	if b.body == nil {
+		b.body = &tx.TxBody{}
+	}
+
+	if b.auxSignerData == nil {
+		b.auxSignerData = &tx.AuxSignerData{}
+		if b.auxSignerData.SignDoc == nil {
+			b.auxSignerData.SignDoc = &tx.SignDocDirectAux{}
+		}
+	}
+}