Support SSZ in the Engine API

[marioevz]

Tracking issue to support ethereum/execution-apis#793 and support clients that want to implement it.

[marioevz]

This alternative was proposed during ACD-C 5-14-2026: ethereum/execution-apis#793

I think there's no difference between on the SSZ side, just how it is handled via REST, so we could start by implementing the SSZ part.

[marioevz]

Deprecation of JSON RPC was mentioned during ACD-C 05-28-26, hence the higher priority now.

[RazorClient]

Hey!
Can you please assign me this issue!