feat(sidecar): add slashing protection append-only commitment log

[merklefruit]

Validator clients rely on accidental slashing protection databases (e.g. lighthouse). This database records every block and attestation signed by validators, and the validator client uses this information to avoid signing any slashable messages.

We should add a similar feature in bolt-sidecar to keep track of signed commitments over time.
For instance, this would be useful if a sidecar accidentally shut down after signing a commitment; upon restart it would know that it has signed a commitment and can recover its pending state to accept new commitments or build a local fallback block if required.

This is a blocker for mainnet, especially related to #541