[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.
The current implementation looks like:
```JavaScript
"_embedded": {
  "details": {
    "attribute": "Subject"
  }
}
```
I guess this matches the requirements stated above.