Expose renku config command through core service

[lorenzo-cavazzi]

Is your feature request related to a problem? Please describe.
The UI now uses configurations stored in the .renku/renku.ini file. Currently, we access them in read-only mode by invoking a GitLab API to read the raw file.
This solution is acceptable, but now we want to provide the users with an interface to modify and remove those values, and we prefer to keep it in line with the renku-python renku config command.

Describe the solution you'd like
A simple API structure that exposes the renku config commands with very similar syntax would be enough.
Here is a proposal [updated]:

• 0.1.0: https://app.swaggerhub.com/apis-docs/lorenzo-cavazzi/renku-core/0.1.0
• 0.1.1: https://app.swaggerhub.com/apis-docs/lorenzo-cavazzi/renku-core/0.1.1 --> set/remove more than 1 key at the same time
• 0.1.3: https://app.swaggerhub.com/apis-docs/lorenzo-cavazzi/renku-core/0.1.3 --> single action to set or remove, remove project_id and use instead git_url + branch , change properties naming

Feel free to point out improvements or changes.

Describe alternatives you've considered
Writing directly to the file through GitLab seems to be the only alternative. We prefer not to go down that way since the UI would have its own custom implementation of renku config.

Additional context
This is the UI reference issue where the discussion started SwissDataScienceCenter/renku-ui/issues/1114
Feel free to chime in! 😃