Produce block v4 with payload

[shane-moore]

Introduces the block production and execution payload envelope endpoints for post-Gloas (ePBS) forks. These were separated out from #552 to be discussed and reviewed independently. Prior discussion in PR 552 thread and shane-moore/beacon-APIs#2 as well.

New endpoints:

• GET /eth/v4/validator/blocks/{slot} — Post-Gloas block production endpoint (produceBlockV4).
  Supports an include_payload query parameter that controls whether the execution payload envelope and blobs are returned inline (stateless multi-BN operation) or cached by the beacon node for separate retrieval (stateful single-BN operation).
• GET /eth/v1/validator/execution_payload_envelope/{slot}/{builder_index} — Fetch a cached execution payload envelope for signing and publishing (used in stateful mode or by builders).
• POST /eth/v1/beacon/execution_payload_envelope — Publish a signed execution payload envelope to the network.

Open Questions

• Whether a BlindedExecutionPayloadEnvelope (omitting transactions) should replace the full envelope in the stateful POST flow for both proposer self-building and builder envelope submission, since the beacon node already has the data cached (see discussion)
• Builder-specific endpoints for constructing envelopes from custom block building (tracked in Builder api to construct envelope from their execution payload #575) and perhaps warrants a separate PR

[nflaig]

while implementing this, I noticed the slot alone is not definitive enough (eg. in case of forks), we should consider also passing the beacon_block_root here