17403 implement collection specification

[``* `#17403` Implement collection specification``](https://community.openproject.org/work_packages/17403)
As those are integration tests, we should IMO test:

* [ ] response successful, although the name does not reflect this. 
* [ ] type of the response "Collection"
* [ ] type of the elements in _embedded/elements, the type of the first should suffice. 
* ~~total, count~~ those are testable on unit test level 

I am aware, that this is a matter of personal taste, but to me this strikes the balance between performance and the need to specify behaviour.