engine: add Rest-SSZ spec

[MariusVanDerWijden]

There have been multiple attempts at this already.
Moving away from JSON-RPC to REST-SSZ.
However most kept the engine api as is.
I think we have a good shot at refactoring the engine api with this change.

This Draft does that; the move and the refactoring.

Happy for any feedback I can get!

The core of the change is:

Old method	New endpoint	Notes
engine_newPayloadV{1..5}	POST /{fork}/payloads	parentBeaconBlockRoot and executionRequests folded into the SSZ envelope; expectedBlobVersionedHashes removed; INVALID_BLOCK_HASH removed from the status enum
engine_forkchoiceUpdatedV{1..4}	POST /{fork}/forkchoice	one atomic call; carries forkchoice state, optional payload_attributes, and (Amsterdam+) optional custody_columns
engine_getPayloadV{1..6}	GET /{fork}/payloads/{id}	poll-style, same semantics as today
engine_getPayloadBodiesByHashV{1,2}	POST /{fork}/bodies/hash	{fork} selects the response schema (not the era of requested blocks); POST because hash lists are too large for URLs
engine_getPayloadBodiesByRangeV{1,2}	GET /{fork}/bodies?from=...&count=...	{fork} selects the response schema
engine_getBlobsV1	POST /blobs/v1	independently versioned; legacy version numbers carry forward
engine_getBlobsV2	POST /blobs/v2	all-or-nothing cell proofs
engine_getBlobsV3	POST /blobs/v3	partial-response cell proofs
engine_getBlobsV4	POST /blobs/v4	cell-range selection
engine_getClientVersionV1	GET /identity + X-Engine-Client-Version request header	unscoped
engine_exchangeCapabilities	GET /capabilities	unscoped
engine_exchangeTransitionConfigurationV1	removed	already deprecated since Cancun