authz: change FetchUserPerms to return RepoIDType
Created by: unknwon
This PRs changes the signature of our authz.Provider.FetchUserPerms
to include an indication of whether the returned list contains exact matches or prefix matches. This change is needed for upcoming Perforce authz provider, which can only provide information via prefix-matching.
Subset of #17881 and extracted here for easier review, and part of #16705.