17228 spent time is not part of wp in api v3

[``* `#17228` Spent time not part of API V3's work package response``](https://community.openproject.org/work_packages/17228)

:exclamation: Remove ``model`` when merging this into dev branch.
@myabc Though I agree with you this mimics the behavior of ``estimated_time`` (see @ulferts' https://github.com/opf/openproject/pull/2191#discussion_r20215728 and the [specs for ``estimated_time``](https://github.com/opf/openproject/blob/release/4.0/spec/lib/api/v3/work_packages/work_package_model_spec.rb#L137-L144)).  So, we should first get to a general agreement, thus, including @NobodysNightmare.