EZP-23418 - The contentTypeGroups REST API's root should be autodiscovered inside the loadContentTypeGroups method

https://jira.ez.no/browse/EZP-23418

Currently, the ``contentTypeGroups`` REST API's root should be provided as a first param of the ``loadContentTypeGroups`` method which is incorrect.

**THINGS DONE:**
- [x] ``loadContentTypeGroups`` method updated,
- [ ] JS unit tests updated.