Allow client-side signing with SIGN_MODE_TEXTUAL

[amaury1093]

Summary

part of #11970

Allow golang clients to be able to generate and sign Txs with SIGN_MODE_TEXTUAL.

Problem Definition

In #13700 we added a SignModeHandler, which means we're able to call GetSignBytes with Textual using our client Tx infrastructure (TxConfig, TxBuilder...).

However, Textual signing is not stateless, and generating the sign doc needs access to latest state (for coin metadata). This issue is about how a client-side Textual can access to state.

For this, we proposed in #13646 to add a context.Context arg to some of the signing functions. Most notably, on the client side, both Sign and SignWithPrivKey take a new context.Context arg.

Proposal

• When passing a Textual instance to the client.Context's TxConfig, we should make so that Textual's coinMetadataQuerier instantiates a bank query client using the grpc.ClientConn, and retrieves the coin metadata via a remote gRPC query.