Group Hooks API not supported #1496

Description of the problem, including code/CLI snippet

The webhook API endpoints for groups appears to be missing from python-gitlab. Only system hooks and project hooks are implemented. Luckily the api is almost identical to that of project hooks, so implementation should be pretty straightforward.

Missing API: https://docs.gitlab.com/ee/api/groups.html#hooks
Project Hooks API counterpart: https://docs.gitlab.com/ee/api/projects.html#hooks

Expected Behavior

To be able to list, get, add, edit and delete group hooks.

Actual Behavior

No API available.

Specifications

• python-gitlab version: v2.7.1
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): 13.11.3-ee