Set XStream mode via http header

Currently kie-server only supports XPath references in XStream. Add a http header to make kie-server's XStream marshaller call:

  xStream.setMode(XStream.ID_REFERENCES);

Motivation by community user:

It's very usefull for projects developed via kie-wb to set XStream mode (ID or ID_REFERENCES) similar to how there's a http parameter to switch between xstream xml, jaxb xml and jackson json.
Actually when submit to kie-server an xml body to post for solution (optaplanner feature) an error com.thoughtworks.xstream.converters.ConversionException: Invalid reference is raised by kie-server, if the xml have reference to facts using id's instead of XPath.
XML with XPath reference is very hard to read for users and to produce dinamically for app that post/get xml data via rest api.