Enhancement: Add stubs and manager for groupprovider service

Recently, there was a separation of concerns with regard to users and groups in
CS3APIs. This PR adds the required stubs and drivers for the group manager.

https://github.com/cs3org/cs3apis/pull/99
https://github.com/cs3org/cs3apis/pull/102
https://github.com/cs3org/reva/pull/1358
