Add endpoint for fetching inclusion lists for a given slot

[conache]

Proposed Changes

• Add GET /eth/v1/validator/inclusion_list?slot={slot} endpoint returning the transactions to be signed by an IL committee member for the given slot.
• This PR stacks on Add support for engine_getInclusionListV1 sigp/lighthouse#9749

Additional Info

• Slot handling: only the IL for the current slot is produced. Any slot provided that is in the past or future causes the endpoint to return a bad request status code.
• Tests: this PR only adds a test for the pre-Heze fork-gate behaviour. Other tests are left as TODO (happy-path of the flow, fetching for a slot other than the current one, EL call failure). These tests will be added in a follow-up PR, after we make sure the harness can operate on a Heze chain. The main blocker is the Heze block production logic, which will be added in a separate PR.