process execution header

[spiral-ladder]

Currently in process_block in the main stf PR, we have process_execution_payload but not process_execution_payload_header.

Spec links:
process_execution_payload: https://github.com/ethereum/consensus-specs/blob/b71a08505246bbc0bac60494e77fa1a2e76f7dca/specs/_features/eip6800/beacon-chain.md?plain=1#L167
process_execution_payload_header: https://github.com/ethereum/consensus-specs/blob/b71a08505246bbc0bac60494e77fa1a2e76f7dca/specs/_features/eip7732/beacon-chain.md?plain=1#L872