Add optional serialization of `Referable.source`-attribute in JSON and XML
It is advantageous to be able to store source information in backends, for example for a Property with a deviating source (e.g. a sensor) compared to the rest of the AAS this property is part of.
Both the JSON and XML (de-)serialization offer the option of adding optional additional content. The JSON schema specifically allows this, while the XML schema allows the adding of attributes to already existing elements. Therefore we would not loose schema compliance by adding the source information to the JSON and XML files.