Remove the need for cristal-integration-rest-server

It shouldn't be necessary to rely on a custom rest endpoint to make Cristal work on XWiki.
Two options need to be considered and discussed:

1. the rest endpoint is not strictly needed and can be replaced with calls to existing rest endpoints from XWiki
2. the rest endpoint is needed and is not covered fully by existing rest endpoints

If it's case 1, the backend code for needs ot be update to use existing rest endpoints.
If it's case 2, the rest endpoints needs to be moved to xwiki-plaform (and the backend code possibly aligned)