Add Support for Pre-Merge Networks

[rzmahmood]

Several pre-merge networks could take advantage of a stack like this.

Currently, when executing the merge on a pre-merge network, the hard fork fails as beaconkit attempts to continue ForkchoiceUpdated from the Genesis file of the execution client. Obviously, this is rejected by the Execution Client as it is an attempt to rollback to a pre-merge block (the genesis block).

Ideally, BeaconKit can pick up after the TDD point is reached. I'm not sure how Mainnet Eth did this, but an idea:

• BeaconKit is started with a flag/config that informs it of the TDD. Beaconkit queries the execution client for the current TDD and employs some strategy to figure out when the TDD is passed. It then continues from there. Strategy would be fairly easy for Clique networks given the constant difficulty of each block.

[imanpajand]

nice idea