feat(l1): add payload witnesses to REST/SSZ Engine API

[jsign]

Note: waiting for the underlying branch PR #6770 to merge first, so for now kept as a draft.

Motivation

Let provers and stateless validators submit a payload and receive its validation status and execution witness in one REST/SSZ request.

Description

Adds authenticated POST /engine/v1/payloads/witness and advertises it through capabilities, implementing execution-apis #885 at commit 40924d4 with support extended to Paris through Amsterdam.

• Returns a complete witness for valid payloads, including repeated submissions, in a two-field SSZ response containing payload_status and witness.
• Returns an empty witness list for other validation outcomes.
• Collects witnesses during fresh execution and retrieves cached witnesses or regenerates them for known payloads.
• Enforces witness limits: 1 KiB per state node or header, 64 KiB per code item, up to 1,048,576 items in each state/code list, and up to 256 ancestor headers.
• Validates ancestor-header chains and fork/timestamp consistency.
• Preserves existing JSON-RPC witness encoding and /payloads behavior.

Validation

• cargo test -p ethrex-rpc --lib: 109 passed.
• cargo test -p ethrex-test --test ethrex_tests rpc::engine_rest_tests: 105 passed, 1 existing test ignored.
• cargo fmt --all --check and git diff --check passed.

[jsign]

cc @iovoid for visibility -- I'll be tracking underlying #6770 and can re-base when appropiate.

[ilitteri]

Retracted.