[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.
@NobodysNightmare I spoke with @myabc about the API's behavior and according to him it should be OK if the API just ignores read-only attributes. This would render this identifier obsolete. What do you think?