SyncCommiteeContribution Aggregators lookahead

[Arkenan]

SyncCommitteeMessages are aggregated by sync committee aggregators. A validator that is in a sync committee can check if it's also an aggregator by ussing get_sync_committee_selection_proof(state, slot, subcommittee_index, privkey) |> is_sync_committee_aggregator(signature). This should be looked ahead of time, as it changes every slot.

specs ref: link