Relay the external builder signature to verifying Web3Signers

[zah]

Besides verifying properties of the signed block, a verifying web3signer may be interested in enforcing a particular white-list of builders (or relays). This is a common practice is a number of Ethereum liquid staking protocols. To support this use case, we'll further extend the verifying Web3Signer protocol with an extra field carrying the signature of builder over the blinded block header.