Support creation and signing of Eth keys

[arajasek]

Related Issues

Not having this functionality was making testing EthAccount abstraction difficult.

Proposed Changes

Support creating Eth keys, and generating signatures with them. Assumes all Delegated keytypes are Ethereum Account senders.

Additional Info

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[raulk]

SGTM.