Provide a lighter REST representation of XObjects

The REST representations of XObjects currently include the object's class metadata in addition to the object field names and values. It results in large REST documents. This issue is about turning the object's REST representation into documents that contain only field names and values. Possible additional field could include the pretty name, and the field type, while leaving all the other metadata to the object's class REST representation itself.

References:

• Discussion on #xwiki, August 20 2019
• Example of an XObject REST representation:
  • myxwiki.org:XWiki.XWikiServerXwiki
  • xwikiserverxwiki-xwikiserverclass.json)