simulators/eth2: Verify block signatures on blocks sent to builder

[marioevz]

Adds a check to verify that all blocks sent to the builder are properly signed by the clients.

Tests will now fail if any block is detected with an invalid signature.

The builder also now returns an error if the signature is invalid, which should help detect this issue in test nets that use the builder as an external command.

Correctly detects prysmaticlabs/prysm#12263

Summary of the issue:

• On their turn to propose, the clients can request a block to be built for them from the Builder API
• The builder API in hive is a mock builder that simply relays blocks built from execution client back to the consensus client as an intermediary
• The mock builder can simulate payloads built incorrectly in order to test that the clients detect these issues, or circuit break the builder altogether in case of an issue
• The client gets a "blinded" payload which is only the header but is missing the full transaction list from the builder
• The client forms the beacon block including this blinded payload, signs it, and sends it back to the builder
• One important step that the hive mock builder was missing from a real relayer (like mev-boost) is that the beacon block signature is validated before returning the unblinded payload
• In reality, if the client delivers a bad signature, the builder will simply refuse to unblind the block and the slot proposal would be missed
• Hive mock builder now checks the signature and expects that the beacon block is well formed and correctly signed