This PR makes EIP-4788 work in the engine API and miner. It also fixes some bugs related to
EIP-4844 block processing and mining. Changes in detail:
- Header.BeaconRoot has been renamed to ParentBeaconRoot.
- The engine API now implements forkchoiceUpdatedV3
- newPayloadV3 method has been updated with the parentBeaconBlockRoot parameter
- beacon root is now applied to new blocks in miner
- For EIP-4844, block creation now updates the blobGasUsed field of the header
This PR moves core/beacon to beacon/engine so that beacon-chain related code has its own top level package which also can house the the beacon lightclient-code.
