16972 add assignee responsible to wp form

[``* `#16972` Support assignee/responsible property on work package forms and PATCH endpoint``](https://community.openproject.org/work_packages/16972)

Includes https://community.openproject.org/work_packages/17426
Right, but if you want to assign the work package to somebody, you must know to whom it is possible. The question, as with all requests for users, is more with regards to the fields returned IMO.