Support block validation before publishing to gossip

[philknows]

Problem description

In order to better support the relayer community, our client would need to support block validation before publishing to mitigate validators sending signed, invalid blocks for equivocation attacks.

In addition, the need to diversify relayer setups for a more robust mev-boost infrastructure is critical as it encompasses ~90% of blocks published on mainnet today.

Solution description

For this feature, we must:

• Validate block first
• Publish to p2p only if valid

This feature should be on a separate Lodestar fork or under a CLI flag for relayers to use.

Additional context

This was already implemented on Prysm and Lighthouse. PRs can be found here:
https://github.com/realbigsean/lighthouse/tree/lighthouse-relay
https://github.com/flashbots/prysm/tree/verify-before-broadcast-v4.0.4

[philknows]

Fixed by #6151