Add inclusion list committee duties Beacon API endpoint

[conache]

Issue Addressed

Implements the inclusion list committee duties endpoint (per Beacon API specs).

Proposed Changes

• Add the POST /eth/v1/validator/duties/inclusion_list/{epoch} endpoint, returning each requested validator's inclusion list committee duty slot
• Compute the duties with a single walk per slot, over the epoch's committees, reusing get_inclusion_list_committee
• The endpoint doesn't reject pre-Heze epochs, matching the other duties endpoints' behaviour. Pre-Heze the response is derived from the same beacon committee prefix the spec uses post-fork, so a validator client polling across the boundary gets a consistent answer
• Add tests for the new endpoint, including fork boundary behaviour checks

Additional Info

The endpoint's response intentionally omits the inclusion_list_committee_root in the returned duty object, even though the beacon-APIs spec PR still lists it. I've flagged on the spec PR that this field should be removed.