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.