16971 work package forms endpoint

[``* `#16971` Work package forms endpoint for API V3``](https://community.openproject.org/work_packages/16971)
``_changed?`` returns true if the value has changed but shouldn't actually _change_ the lock version but _set_ it. Thus, with ``ActiveModel::Dirty`` we're are not able to say if a attribute was just touched (without changing it). So, the short answer is (unfortunately) no.