django/utils: Add KeycloakOpenIDConnect factory function
Add keycloak_oidc_client factory to instantiate KeycloakOpenIDConnect class from the following django settings:
- KEYCLOAK_SERVER_URL
- KEYCLOAK_REALM_NAME
- KEYCLOAK_CLIENT_ID
This is required before moving the generic django OIDC auth backends from swh-web to swh-auth.
Migrated from D5318 (view on Phabricator)