Support group-level service-accounts

[sven-hergenhahn-dmtech]

Since 16.1, the gitlab-api supports the creation of group-level service-accounts and group-level service-account tokens (They are not the same as group access tokens!)
So far they can not yet be created via the Web UI.

It would be great to have this feature added to python-gitlab.

The gitlab API documentation is here: https://docs.gitlab.com/ee/api/groups.html#service-accounts