Make `VALIDATOR_REGISTRATION_EXPIRATION_EPOCHS` configurable

[tbenr]

With regard to #6237 it might be useful for some deployment to configure the latency in which builder usage for a given public key is actually deactivated on BN. In case users are using proposer config file refresh option (and in future with an additional VC rest API), they could dynamically set builder.enabled to false.

VALIDATOR_REGISTRATION_EXPIRATION_EPOCHS is currently governing how quickly BN reacts to that by expiring he validator registration.

Making it configurable with a BN parameter users can have adjust it if they want to.