16770 work package api patch for subject

[``* `#16770` Work package API PATCH for subject``](https://community.openproject.org/work_packages/16770)

This PR applies the specified behavior outlined in #2056.
How about adding specific representers then with the identifiers included. Might seem a bit elaborated at the beginning but the way I see it, you [are already repeating case statements](https://github.com/opf/openproject/pull/2081/files#diff-d5b7d1e80a5236e2a654ba407f1a8643R72).