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

- [x] Path service
All of those methods have ```api_v3_<unique>_path```. How about changing this so that the call is ```api_v3_path.<unique>```?