engine: add POST /{fork}/payloads/witness to the REST+SSZ refactor

[developeruche]

What this does

Adds the witness route to the REST + SSZ Engine API refactor:

POST /engine/v2/{fork}/payloads/witness: same request as POST /{fork}/payloads, but the response is a superset of PayloadStatus that also carries the stateless ExecutionWitness. It folds the legacy two-call flow (engine_newPayload → debug_executionWitness) into a single round-trip.

This integrates the proposal from ethereum#773 into this draft, per the conclusion reached on the ACD call and in the threads below. It replaces ethereum#773's standalone POST /new-payload-with-witness endpoint with one that follows this spec's resource model and SSZ conventions.

Changes

refactor.md

• New "Payload submission with witness" section defining the endpoint
• Rows added to the old → new mapping table and the resource model table
• TOC entry

refactor-ssz.md

• ExecutionWitness shared container (state / codes / headers as ByteLists)
• PayloadStatusWithWitness response container for the endpoint
• MAX_WITNESS_ITEMS / MAX_WITNESS_ITEM_BYTES constants
• TOC entries

Related

• Supersedes Introduction of REST based + SSZ serialized new-payload-with-witness ethereum/execution-apis#773 (which can be closed once this lands)
• Context: engine: add Rest-SSZ spec ethereum/execution-apis#793 (comment), Introduction of REST based + SSZ serialized new-payload-with-witness ethereum/execution-apis#773 (comment), and the ACD call notes

cc @MariusVanDerWijden @LukaszRozmej @RazorClient