Implementation of EIP-7742 for Prague

[jangko]

TODOs:

• How maximumBlobCount should be used in block creation?
• Where targetBlobCount come from when converting to Header. From ExecutionPayloadV4 or from engine_newPayloadV4's param?
• What about engine_getPayloadV4? should the response return ExecutionPayloadV4 or additional targetBlobCount field?
• Difference between EIP-7742 spec targetBlobCount of block header, and consensus spec targetBlobsPerBlock of PayloadAttributesV4
• Upgrade on nimbus-eth2 side

fix #2753

[tersec]

TODOs:

* [ ]  How `maximumBlobCount` should be used in block creation?

My understanding is that it's exactly the same as the current max blob count today (6), but not a constant across blocks.

* [ ]  Where `targetBlobCount` come from when converting to `Header`. From `ExecutionPayloadV4` or from `engine_newPayloadV4`'s param?

This is being actively discussed in the EIP-7742 - EL and CL candidates thread in the #allcoredevs channel of the Eth R&D Discord server.

* [ ]  What about `engine_getPayloadV4`? should the response return `ExecutionPayloadV4` or additional `targetBlobCount` field?

As far as I can tell, undetermined as of yet.