Gloas Signing Support

[StefanBratanov]

Gloas introduces three new objects that needs to be signed.

• execution_payload_bid
• execution_payload_envelope
• payload_attestation_message

https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/builder.md
https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/validator.md