tx-signer: Ledger support

[tony-iqlusion]

This is a tracking issue for supporting account keys held by a Ledger device.

Since Ledger apps which sign Tendermint transactions today (e.g. the Cosmos App) typically require user interaction, and KMS transaction signing is specifically intended to be used for online, non-interactive signing, the first step towards adding support for this feature would likely involve adding account key support to the Ledger Tendermint Validator App which provides non-interactive account key support.

[assafmo]

How will that work? How can you go around the physical requirement of a user clicking on the device?

[tony-iqlusion]

The Ledger Tendermint Validator App already performs non-interactive Ed25519 signing for consensus keys.

It could potentially be extended to also support non-interactive ECDSA/secp256k1 signing for account keys.

Granted in either case, boot is attended and the user must enter their pin before launching the app.

[tony-iqlusion]

tx-signer support was removed in #764