Isolate REST API JAXB model classes in a separate module

Currently the JAXB model classes for handling XML representations of the resources available in the REST API are in the xwiki-rest module. This means that a client that wants to use JAXB to parse and send XML representations to the REST API has to declare a dependency to the xwiki-rest module and drag in the client all its dependencies (i.e., xwiki-core, restlet, and so on)

By isolating the JAXB model classes in a separate module, clients will be able to use them without having additional dependencies included.