Support multiple keys within signer

[cmwaters]

Summary

Kinda doesn't need one

Problem Definition

Currently, the signer can only be set with a single address that it uses to sign all the transactions. Some users may want to submit sdk.Msgs with different accounts. This is the case with Raas providers who may want a dedicated account per rollup.

Proposal

Each transaction has a concept of signers. What is needed is that the signer needs to be read from the transaction and then checked whether the keyring has it. If so we sign with that account. We still won't support transactions with multiple signers but this is a rare requirement. For PFBs the signer will need to be provided. It may be possible to do this as an optional argument (especially if we know that the keyring only has a single signer).