stdtx: tracking issue for Protobuf transaction support

[tony-iqlusion]

Cosmos SDK is migrating to a new transaction format based on Protocol Buffers, which is tracked in the following issue:

cosmos/cosmos-sdk#6213

Presently the stdtx crate implements transaction signing using legacy Amino formats (JSON + binary serialization).

This issue is for tracking planning/work on migrating the stdtx crate to use the new Protobuf-based transaction formats being implemented upstream in the Cosmos SDK.