Add a mechanism to modify the beacon root

[fvictorio]

Add an initialParentBeaconBlockRoot option to the configuration of the Hardhat Network, and a hardhat_setNextBlockParentBeaconBlockRoot JSON-RPC method.

Search terms: parentBeaconBlockRoot, EIP-4788, EIP4788, 4788

[naddison36]

This would be very useful for testing merkle proofs against beacon block roots