Get the DKIM selector index

The compact index used to name the DKIM keys already published for a domain. A key is matched either by selectors, the selector conventions a source publishes under, or by base_domains, the zones its CNAME targets sit in — the latter being the only signal that identifies a per-customer selector such as HubSpot's hs1-<portalId>. Only sources carrying at least one of the two appear. Authentication is optional, as on the list endpoint.