16971 work package forms endpoint

[``* `#16971` Work package forms endpoint for API V3``](https://community.openproject.org/work_packages/16971)
This is [implemented in the contract](https://github.com/opf/openproject/blob/0c0cdb4f1ebab24b196414e581c599e2a2ca5169/lib/api/v3/work_packages/work_package_contract.rb#L57-L62). See https://github.com/opf/openproject/commit/0c0cdb4f1ebab24b196414e581c599e2a2ca5169 for the related changes and https://github.com/opf/openproject/commit/b72d0e22022024644262258eb76a11cc5f108f58 for the related specs.