Add configurable subject_identifier_uri to clients

According to the [OIDC
Documentation](https://openid.net/specs/openid-connect-core-1_0.html#PairwiseAlg),
the client must configure a `subject_identifier_uri`, if multiple
`redirect_uri` are configured.

This patch adds the possibility to do this as described in #49.

Signed-off-by: Sven Haardiek <sven@haardiek.de>

Author: shaardie

src/pyop/provider.py

@@ -240,7 +240,8 @@ def _create_subject_identifier(self, user_id, client_id, redirect_uri):
         """
         supported_subject_types = self.configuration_information['subject_types_supported'][0]
         subject_type = self.clients[client_id].get('subject_type', supported_subject_types)
-        sector_identifier = urlparse(redirect_uri).netloc
+        sector_identifier_uri = self.clients[client_id].get('sector_identifier_uri', redirect_uri)
+        sector_identifier = urlparse(sector_identifier_uri).netloc
         return self.authz_state.get_subject_identifier(subject_type, user_id, sector_identifier)
 
     def _get_requested_claims_in(self, authentication_request, response_method):