enterprise: Add support for sub-repository syncing via PermsSyncer
Created by: ryanslade
This change assumes that the data returned by fetchUserPermsViaExternalAccounts
is correct and simply writes the raw sub-repository matching rules to the DB.
For now, it performs one write per repo but we can potentially do this in batches in future.
Closes: https://github.com/sourcegraph/sourcegraph/issues/26643