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)

Please use the following code for merging the representer property into ``dev`` branch (but don't forget to test it):
```Ruby
property :spent_hours:, as: :spent_time, writeable: false
```
This is inconsistent with ```estimated_time``` which additionally provides the units. Any thoughts on that?