Add IL publishing wiring to the BN

[conache]

Issue Addressed

This PR wires up the IL publishing endpoint on the BN side, which the VC uses to publish signed inclusion lists (per spec).

Endpoint: POST /eth/v1/validator/inclusion_list per beacon-api spec.

Proposed Changes

• Add the POST validator/inclusion_list endpoint (JSON and SSZ variants) and the corresponding BeaconNodeHttpClient methods
• The endpoint rejects while syncing (503), validates the Eth-Consensus-Version header, and rejects inclusion lists for pre-Heze slots (400)
• On successful verification, the inclusion list is broadcast as a PubsubMessage::InclusionList

Additional Info

• this PR depends on:
  • Wire the inclusion_list gossip topic sigp/lighthouse#9846
  • Add base support for Heze block production sigp/lighthouse#9714
• for now, the gossip verification logic is intentionally a stub. GossipVerifiedInclusionList::new currently accepts every inclusion list; the p2p-interface checks land with the IL gossip verification follow-up, which should also fill in the remaining verification error variants
• The test helper derives the IL committee via get_beacon_committees_at_slot, which should be replaced with get_inclusion_list_committee once the inclusion list committee derivation PR is merged