In order for validators to withdraw staked funds, they need withdrawal credentials containing an execution layer address (and `0x01` prefix).
https://lighthouse-book.sigmaprime.io/voluntary-exit.html#1-how-to-know-if-i-have-the-withdrawal-credentials-type-0x01
The current tooling creates validators with credentials derived from a BLS signature using the initial keypair (with `0x00` prefix). To allow withdrawals, these then have to be updated on the running chain in a separate signed message. By creating validators with EL address credentials at genesis, we avoid this extra step.
Reviewed-on: #1
