16971 work package forms endpoint

[``* `#16971` Work package forms endpoint for API V3``](https://community.openproject.org/work_packages/16971)
I think we need a ```get``` too in order to get the client started.

Additionally, there should be a link to the form in the /work_packages/:id.json response. See:
https://github.com/opf/openproject/blob/dev/doc/apiv3-documentation.apib#L1440