UUID services in GATT should be either 16-bit or 128-bit. Current
GATT interface does not allow to provide 128-bit ones. This patch
enables plugins to register services using 128-bit UUIDs.
Service plugins using the new GATT API may need to know if their
attributes were successfully registered. In this way, plugins might
abort loading operation if they weren't registered.
This new internal API can be used for implementing GATT services (server
role). It is built around the current attribute API (attrib_db_add() and
others).