Feature external signer

[ak88]

Adds clef as a remote signer. https://geth.ethereum.org/docs/tools/clef/introduction

Changes

• new ClefSigner that can communicate using JsonRpc
• CliqueSealer will seal with remote signer if available
• new flag --Mining.Signer for setting url of the remote signer
• using flag KeyStore.BlockAuthorAccount will attempt to sign with that account if available on remote

What types of changes does your code introduce?

• [ ] Bugfix (a non-breaking change that fixes an issue)
• [x] New feature (a non-breaking change that adds functionality)
• [ ] Breaking change (a change that causes existing functionality not to work as expected)
• [ ] Optimization
• [ ] Refactoring
• [ ] Documentation update
• [ ] Build-related changes
• [ ] Other: Description

Testing

Requires testing

• [x] Yes
• [ ] No

If yes, did you write tests?

• [x] Yes
• [ ] No

Documentation

Configuration of clef and using the correct flags needs to be documented. https://geth.ethereum.org/docs/tools/clef/clique-signing#prepping-clef

Requires documentation update

• [x] Yes
• [ ] No

[ak88]

Wny not implement this as a plugin instead. does not feel like it should be part of the main client

Good idea - i will refactor it into a plugin instead.