[Feature Flag] detect possible surround votings

[potuz]

Add a feature flag to let the beacon node act as a lazy slasher, it tracks attestations it has seen as a whole, not discriminating by validator index. If it finds a possible surrounding, get all blocks from the corresponding epochs and produce slashings using #15068 and broadcast the slashings.

Warning: study the design to make it less prone to abuse: any validator sending a bad attestation for a bad target and not attesting the next epoch would avoid being slashed and would trigger nodes to look for slashings. Filtering by number of indices is an option.

[anushkasomani]

hi @potuz , i'd like to work on this issue.