Implementing aggregation endpoint(s) for /contributions

[FabiKo117]

This continues with the original idea about the /contributions endpoint defined in https://github.com/GIScience/ohsome-api/issues/23.

• [x] needs https://github.com/GIScience/ohsome-api/issues/117 to be solved

A /contributions/count endpoint would be useful e.g. for the last edit indicator in the OQT, where the number of edits (=contributions) is the required measure.

• [x] /contributions/count :arrow_right: #121
• [x] /contributions/count/groupBy/boundary :arrow_right: #217
• [ ] /contributions/count/groupBy/contributionType
• /contributions/count/groupBy/_additional_grouping_type(tbd)

[FabiKo117]

Parts of the here mentioned features will be implemented with #121. Therefore, I'll remove the 1.4 milestone for now while still letting the issue open, as the other endpoints will most likely follow soon.