Add Beacon API endpoints for EIP-8025 execution proofs

[frisitano]

Add Beacon API endpoints for EIP-8025 execution proofs

Adds Lighthouse Beacon API server and client support for submitting and retrieving execution proofs, on top of optional-proofs-gloas (base 535046063).

Endpoints

• POST /eth/v1/beacon/execution_proofs — accepts a JSON array or SSZ List[SignedExecutionProofEnvelope, MAX_EXECUTION_PROOFS_PER_PAYLOAD=4]. Each proof runs the same verification as the execution_proof gossip topic, is published to the network, and is cached in the pending payload cache, importing the payload envelope when the proof gate is satisfied.
• GET /eth/v1/beacon/execution_proofs/{block_id} — returns the proof envelopes cached for a block's payload, keyed by beacon block root and ordered by proof type, as JSON ({ execution_optimistic, finalized, data }) or SSZ.

Client

BeaconNodeHttpClient gains post_beacon_execution_proofs[_ssz] and get_beacon_execution_proofs[_ssz] over the shared SignedExecutionProofEnvelopes wire type (no duplicated structs).

Response codes

200 when every proof is on the network (published now or already known); an indexed 400 naming the proofs gossip would drop; 404 for unknown blocks; 500 for proof engine or beacon chain faults; 501 on nodes without --proof-engine.

Contract decisions (for the beacon-APIs PR)

• Request body is a bare list, matching the other batch publish endpoints, not { "proofs": [...] }.
• Wire type is the gossip envelope SignedExecutionProofEnvelope, never re-signed, adapted from the older SignedExecutionProof + PublicInput shape.
• proof_type serializes as a quoted string in JSON to match the beacon-APIs Uint8 schema; SSZ and gossip encoding are unchanged.
• MAX_EXECUTION_PROOFS_PER_PAYLOAD = 4 taken from the EIP text.

Validation

• cargo fmt --all -- --check: clean.
• cargo test -p types execution_proof (7), -p warp_utils (3), -p beacon_chain --lib pending_payload_cache (12), -p eth2 (13): pass.
• cargo test --release -p http_api --test bn_http_api_tests execution_proof_tests (3): pass — JSON and SSZ submit/get, publication, lookup by root/head/slot, 404, 501, indexed 400 failures.
• cargo clippy on the five changed crates with the make lint flags: no issues.
• Not run: full bn_http_api_tests suite, the FORK_NAME matrix, workspace clippy, network/Kurtosis.

Note

Retrieval currently reads the in-memory pending payload cache (last 32 payloads); a follow-up persistent-storage task will extend this beyond recent blocks.