Determine if a distributed validator has been selected to make a sync committee contribution.
Submit sync committee selections to a DVT middleware client. It returns the threshold aggregated sync
committee selection. This endpoint should be used by a validator client running as part of a distributed
validator cluster, and is implemented by a distributed validator middleware client. This endpoint is
used to exchange partial selection proofs (slot signatures) for combined/aggregated selection proofs to
allow a validator client to correctly determine if any of its validators has been selected to perform a
sync committee contribution (sync aggregation) duty in a slot. Validator clients running in a distributed validator cluster must query this endpoint
at the start of each slot for all validators that are included in the current sync committee. Consensus
clients need not support this endpoint and may return a 501.