[APIv3] Specify first iteration of WorkPackage PATCH-Endpoint

This PR belongs to [#16770](https://community.openproject.org/work_packages/16770).

It defines the very first portion of the `PATCH` endpoint for work packages.
This is not implemented at the moment (it was and still is the client code that introduces the punctuation - see https://github.com/opf/openproject/commit/8629bd9ecc6db209ce8ecc99ccfb362c7022ef92).