Use Builder Pubkey instead of Builder Index

In the RequestAuth, we currently sign over the builder index. Builder indices are only known for builders after the Gloas fork transition which will require builders to advertise their indices once they have been registered on-chain at the fork.

If we use the Builder pubkey instead, builders can advertise the pubkey with which they will become an on-chain builder before the fork transition. Validators can then update their configurations with the builder pubkey before the fork and not have to rely on the builder having to advertise their index after the fork.