Add genesis fork version to ValidatorRegistrationSigning

[jshufro]

In implementing the ValidatorRegistrationSigning portion of this API, I discovered that the signing domain is computed from:

• domain_type: application/builder - a constant
• genesis fork version - unique to a chain
• 96 byte unset buffer (in lieu of genesis validator root) - a constant

This is distinct from other types, where the domain is computed from:

• domain_type
• fork version for the epoch relevant to the container being signed
• genesis validator root

It seems like we deliberately use a mask based on a chain id when setting fork versions, so the genesis fork version is sensibly unique to a specific network.

Adding the genesis_fork_version to ValidatorRegistrationSigning as an optional (at first) field provides a clean migration path across versions.

[eth2353]

I'm not against this but Gloas will come with some big changes related to validator registrations / proposer preferences that may require a bigger change in the Remote Signing API. We should probably wait and see how ethereum/beacon-APIs#570 plays out. I don't think it would be worth making changes to this endpoint for Fusaka only.

[jshufro]

Converting to draft until gloas solidifies