Compare validation logic to existing one and extend where required

[ckoopmann]

Unlike the existing endpoint this validation method only uses the ExecutionPayload and ignores the "Message" and "Signature". Probably have to extend the logic to also verify the signature. (if that is part of the expected behaviour).

Are there any other validation steps that we currently do that are not included in here yet ? (haven't really compared the implementation details of the full_validation method here with the reference implementation here).