Withdrawals: Standardizing wei --> gwei between the EL and CL

[non-fungible-nelson]

https://notes.ethereum.org/@ralexstokes/withdrawals-in-gwei

Need to update the Engine Spec and work on EIP-4895 to use Gwei (which the CL uses) vs Wei that we use now. See the above spec changes.

Spec changes of note:

Engine API - the amount value is represented on the beacon chain as a little-endian value in units of Gwei, whereas the
amount in this structure MUST be converted to a big-endian value in units of Gwei.

Withdrawals POC - "4. a nonzero amount of ether given in Gwei (1e9 wei) as a uint64 value.
Recall that the amount is given in units of Gwei so a conversion to units of wei must be performed when working with account balances in the execution state."