engine: align WithdrawalRequestV1 with EIP-7002 and consensus spec

[nflaig]

There is an inconsistency between the withdrawal request defined in EIP-7002 (see validator_pubkey field and related PR), the consensus spec (see ExecutionLayerWithdrawalRequest), and the execution api.

This PR align the execution api spec WithdrawalRequestV1 with the other specs.

Most ELs currently return it as validatorPublicKey with the exception of Besu which uses validatorPubKey

[mkalinin]

I like the alignment part but the renaming can cause interop breaks between CL and EL. So we should be careful about when to introduce this change on the client side, at least it should be after devnet-0

[nflaig]

Do we wanna include this for devnet-1? Might need more visibility on it first, I am not that familiar how you do this on the execution api side but maybe could bring this up in discord and see if there is any opposition to the change