chore: remove builder_index from produce_block_v4

[shane-moore]

Issue Addressed

Part of #8828 for the stateful path and helps align gloas produceBlockV4 with beacon-APIs PR

Proposed Changes

• Plumb include_payload query through the handler. Ignored for now since stateless mode isn't wired up yet
• Add execution_payload_included metadata field + Eth-Execution-Payload-Included header per spec. Both false until stateless lands
• Drop the {builder_index} segment from the envelope GET URL since no longer included in spec

Additional Info

First of a few PRs here. Follow-ups will rekey the BN envelope cache by beacon_block_root and bring the POST envelope endpoint up to spec