Like OptaPlannerJacksonModule, introduce a "one line registration" for all XStream and JAXB convertors

In exe server, the XStreamMarshaller already does:

AbstractScoreXStreamConverter.registerScoreConverters(xstream);

But that's not really consistent with how jackson's stuff gets registered:

        objectMapper.registerModule(OptaPlannerJacksonModule.createModule()); // Not just score, also constraintmatch etc in the future

And jaxb doesn't seem to have anything at all, see JaxbMarshaller in exe server and see the lack of a register all like method. But IIRC, JAXB doesn't even support doing any such registration to begin with...

Reqs:

• Create something like OptaPlannerXStreamModule.register() in optaplanner-persistence-xstream jar (consistent with the jackson one), and remove AbstractScoreXStreamConverter.registerScoreConverters().
• Research if you can do something similar for jaxb
• Adjust integration chapter docs in optaplanner-docs accordingly (see jackson section approach)