feat: EthApi traits abstraction

[klkvr]

This completes most of the abstraction for eth namespace traits.

Changes include:

• Header generic is added to EthApiServer
• LoadPendingBlock is refactored so that header and block construction are handled by a separate assemble_block method and can be defined separately for OP and eth. Eventually this should be refactored to use payload builder instead

This should unblock abstraction of other namespaces implementations. The latest not abstracted endpoint for eth namespace is eth_simulateV1 which also requires building blocks, I'll work on it in a separate PR

[mattsse]

this is fantastic,

there are things we can try to make nicer soon, but for now this is a big unlock