Provide a converter from XWiki JSON to Cristal JSON-LD

The architecture choice is to use JSON-LD to represent and store documents in Cristal. Hence we need a converter from XWiki JSON to Cristal JSON-LD. Alternatively (better), XWiki should be modified to be able to output directly JSON-LD from its REST calls.

Note: this issue will depend in we choose JSON-LD as the internal format in memory or not. If it's another format we'll still need to provide a converter for it.