Builder API: Validator Registration

[ciaranmcveigh5]

To be used by Obols Charon client

Validator clients have to register with a relay to gain access to the builder network. They are recommended to re-register every epoch.

The "validator registration" section reference in "Builder -- Honest Validator" has additional details

https://github.com/ethereum/builder-specs/blob/main/specs/validator.md#validator-registration

The specs for the payload that is sent with this registration can be found in the link below (ValidatorRegistrationV1 & SignedValidatorRegistrationV1)

https://github.com/ethereum/builder-specs/blob/main/specs/builder.md#validatorregistrationv1

Happy to draft up a rough PR using "BlindedBeaconBlock" references in the repo as a template

Have made a start at implementing the changes on #19

Thanks