Support multi signers in `@cosm/stargate`

The current sign() function of SigningStargateClient accepts only one signerAddress, but Cosmos basically support multi-signers. https://github.com/cosmos/cosmjs/blob/06fbc34f72f12c30a396c3ca296f80eca9fa60b0/packages/stargate/src/signingstargateclient.ts#L301-L307

I’d like to suggest to create a new signWithMultiSigners() function that looks like below:

public async signWithMultiSigners(
    signerAddresses: string[],
    messages: readonly EncodeObject[],
    fee: StdFee,
    memo: string,
    explicitSignerDatas?: SignerData[],
  ): Promise<TxRaw>

I’ve already implemented this function by following the Cosmos’ guide, so that I can use it for my Cosmos-based blockchain. I think this is widely useful. If you think it’s reasonable, I’m ready to open a PR 😃

[webmaster128]

Re-opening as the request is very valid. We can implement a solution that includes the use case described here but supports a multi-machine setup ad well. Many of the changes from #838 make sense, specially looking at the signture of makeAuthInfoBytes.

[youngjoon-lee]

Yeah. That is an important first step, as you said. Thank you for handling this issue.