support calling engine api

[ncitron]

For integrating with EL full nodes, we should support calling the engine api, specifically engine_forkchoiceUpdatedV1, which Akula uses to follow the chain.

[dadepo]

Just to be clear, the purpose is to be able to drive the EL by making this call? What about the engine_newPayloadV1 call. Should that not also be included?

[ncitron]

Yep. Eventually we probably want to support all the parts of driving the execution layer, but engine_forkchoiceUpdatedV1 is a good one to start with because it turns out Akula only uses that one.

We also probably want to both drive the API directly, and to provide a separate interface (maybe using tokio channels), so that this could be built into Akula directly with no API intermediary needed.