Doppelgänger Protection Feature

[philknows]

Is your feature request related to a problem? Please describe.
Feature Request as part of #3687 RocketPool Integration and safety for validators.

Describe the solution you'd like
Reference implementation from Lighthouse: https://lighthouse-book.sigmaprime.io/validator-doppelganger.html

Miss 2 attestations upon startup and listen for attestations coming from the loaded keys, disable validation duties if found because it would be a slashable offense to double attest.

• Expose liveness API inspired by Lighthouse's implementation
• Add the doppelganger service which polls BN's liveness API and keeps info about validators
• Use the doppelganger in validator and check its doppelganger state before performing any slash-able action
• Reasonable test coverage
• Add metrics