tx-signer: post-Stargate roadmap

[tony-iqlusion]

This is a tracking issue for improvements to the tx-signer feature which are possible following the Stargate upgrade.

• Protobuf Transaction Support in stdtx crate: stdtx: tracking issue for Protobuf transaction support crates#459
• gRPC Transaction Sources: tx-signer: gRPC support for Transaction Sources #97
• Replace [[tx_signer.seq_file]] with on-chain query

This roadmap is still TBD, so please subscribe for updates!