Make usage of ExternalObjectReference more precise

Problem Statement

An ExternalObjectReference carries only the "name" attribute, but it is unclear, what the "unique identifier" in case of OpenDRIVE shall be. It could either be the "name" of <object> and <signal> in OpenDRIVE or it could be the "id". Neither of them is strictly unique as "name" can be defined arbitrarily and "id" must only be unique among either <signals> or <objects>, e.g. it is valid to have a signal with "id=1" and an object with "id=1".

Possible Solutions

As in the "name" attribute of TrafficSignalController ("[...], e.g. a OpenDRIVE controller ID."), give a hint for implementations to use the OpenDRIVE id (if that is what is desired, but I think most likely it is). Furthermore, add a second field e.g. "type" to specify if the referenced external object is of type "signal" or "object".