ifp: extraAttributes is UnknownProperty
Attempting to get extraAttributes via SSSD's ifp fails.
Here I uncomment interface function for extraAttributes.
also right for querying this interface is changed to allow
this request.
Resolves:
https://pagure.io/SSSD/sssd/issue/3906
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>