feat: assets endpoints

[TarikGul]

Assets Endpoints

closes: #491
closes: #545

GET assets/{assetId}/asset-info

Associated Controller: AssetsController
Associated Service: AssetsService
Response:

{
    at: { ... }
    assetInfo: {
        owner: AccountId,
        issuer: AccountId,
        admin: AccountId,
        freezer: AccountId,
        supply: TAssetBalance, // U64
        deposit: TAssetDepositBalance, // BalanceOf
        minBalance: TAssetBalance,
        isSufficient: bool, // polkadot-js way of saying boolean
        accounts: U32,
        sufficients: U32,
        approvals: U32,
        isFrozen: bool
    }
    assetMetadata: {
        deposit: TAssetDepositBalance,
        name: Bytes, // Polkadot-js type
        symbol: Bytes,
        decimals: U8,
        isFrozen: bool
    }
}

AssetInfo(AssetDetails): substrate ref | polkadot-js ref
AssetMetaData: substrate ref | polkadot-js ref

GET accounts/{accountId}/asset-balances?assets=assetId

Notes:

1. Originally per the issue filed above, there were two different proposed responses depending on whether an assetId is provided as a parameter. I changed it slightly to always have one type of response. The only difference is when a assetId is provided the assets array that is returned will only have the values provided in the query param. This choice was made to simplify return values for the end user. Happy to change this or discuss suggestions.

2. assetId query parameter is treated as an array. It is formatted as such: assetId=id1,id2,id3

Associated Controller: AccountsAssetsController
Associated Service: AccountsAssetsService
Response:

{
    at: { ... },
    assets: [
        {
            assetId: AssetId,
            balance: TAssetBalance,
            isFrozen: bool, 
            isSufficient: bool
        }
        ...
    ]
}

AssetBalance: substrate ref | polkadot-js ref

GET accounts/{accountId}/asset-approvals?assetId=assetId&delegate=accountId

Note: Both a delegate and assetId are required for this endpoint. The owner => accoundId and the delegate can be referred to as the AssetApprovalKey in polkadot-js or ApprovalKey in substrate.

Associated Controller: AccountsAssetsController
Associated Service: AccountsAssetsService
Response:

{
    at: { ... },
    amount: TAssetBalance,
    deposit: TAssetDepositBalance
}

AssetApproval: substrate ref | polkadot-js
AssetApprovalKey: substrate ref | polkadot-js

TODO's

• Asset Endpoints
• Unit tests
• Manually test all cases against statemint
  • assets/:assetId/asset-info
  • accounts/:address/asset-balances
  • accounts/:address/asset-balances?assets[]=10&assets[]=1
  • accounts/:address/asset-approvals (Fails correctly, this is subject to change)(delegate, and assetId might be part of the path)
  • accounts/:address/asset-approvals?assetId=assetId&delegate=delegateId
• Docs
  • inline docs
  • swagger ui docs

Other

1. Important to note I added statemint's metadata to the metadata library. It was the only way for me to have access to the assets methods inside of testing.

[emostov]

Looks good! I just have one suggestion for how to change the api for fetchAssetApproval that I think will improve readability of the None response. (And tiny recs in docs)

[dvdplm]

lgtm, modulo nitpicks.

[dvdplm]

I'm fine either way but I don't mind long test files generally. They are typically not read top to bottom anyway?

[TarikGul]

This PR is ready to merge. Just manually tested against Statemint today again. In return we found out the version of polkadot needed to be updated in order to resolve a few more types. I also changed all the pattern fields for accountId in the docs to format: SS58 in order to close #545.