Support custom status in REST

Right now you can get the status of a job trough REST but you only get what is provided by org.xwiki.job.event.status.JobStatus.

The problem is that many job statuses also expose custom methods which are important. For example the install plan when you execute install plan job. If you can't access it the status is pretty much useless.

The difficulty is find how to serialize custom values.
Here are a few ideas:

• allow any extension to customize the serialization of the job status REST POJO (i.e. produce an extended instance of org.xwiki.rest.model.jaxb.JobStatus)
• introduce Map<Strin, Object> like properties in JobStatus which to expose public data to serialize in the REST POJO (like we do for the job request in XWIKI-12136)