[Feature] 14005 show costs attributes in overview tab

[``* `#14005` Show costs attributes in overview tab``](https://www.openproject.org/work_packages/14005)

The following points are still missing:
- [x] Remove ``:spent_time`` in [``work_packages_overview_attributes.rb``](https://github.com/finnlabs/openproject-costs/blob/feature/14005_show_costs_attributes_in_overview_tab/lib/open_project/costs/hooks/work_packages_overview_attributes.rb) (solved in [Core commit 73bcf53](https://github.com/opf/openproject/commit/73bcf53683c7ed55f88cecbccae570c020ae55af))
- [x] Add overall costs information including the currency to API v3
- [x] Add spent units (a list of sums and their units) to API v3
@hschink Shouldn't the self link be pointing to an API endpoint?