Provide beacon node with registrations for the given validators to the external builder network.

Prepares the beacon node for engaging with external builders. The
information must be sent by the beacon node to the builder network. It is
expected that the validator client will send this information periodically
to ensure the beacon node has correct and timely registration information
to provide to builders.

Note that only registrations for active or pending validators must be sent to the builder network.
Registrations for unknown or exited validators must be filtered out and not sent to the builder network.