Client: Add PARALLEL and VOLATILE categories

I'm not 100% sure what's appropiate for plproxy functions so, for now,
I've set them up as VOLATILE and PARALLEL UNSAFE

client/sql/15_config_management.sql

@@ -34,4 +34,4 @@ BEGIN
     result.organization_name = organization_name;
     RETURN result;
 END;
-$$ LANGUAGE 'plpgsql' SECURITY DEFINER;
+$$ LANGUAGE 'plpgsql' SECURITY DEFINER STABLE PARALLEL SAFE;