Full payload building

[michaelsproul]

It might be useful for eleel to build non-dummy payloads. One strategy would be to require the controlling CL to build a payload, and then serve this to all requesters

[michaelsproul]

We have something like this now, but the payloads are invalid post-Capella due to the presence of withdrawals (we can't calculate the state root).

A better solution would be to integrate Besu's engine_preparePayload method, or maybe something from Reth

• Besu: add engine_preparePayload_debug endpoint hyperledger/besu#4427