tx-signer: Ethereum transaction support

[tony-iqlusion]

This is a tracking issue for potentially adding support for Ethereum transactions.

We are considering using the ethers.rs library to produce Ethereum transactions in a similar manner to how we use the stdtx crate to produce Tendermint transactions.

Ideally we can support both YubiHSM2 and Soft Sign backends for this purpose, and potentially Ledger as well.

[tony-iqlusion]

tx-signer support was removed in #764