authz: Add a basic SubRepoPerms client
Created by: ryanslade
This is a skeleton implemenation intended to allow to start plugging it into all the places where we intend to check sub repo permissions.
The SubRepoPermsStore was moved from enterprise to OSS since only the actual PermSyncer needs to stay enterprise.
Part of https://github.com/sourcegraph/sourcegraph/issues/26645