This change set adds specs for the Groups Rest API to test the avatar upload.
More importantly though it changes the avatar upload to be handled by workhorse. This is mostly as a preparation for allow to remove group avatars via Rest API, similar to what will be possible for Projects with !92604 (merged) and which is already possible for Topics with !73969 (merged)
A follow up MR will implement the avatar removal and the same for the Users API.
The main trigger for this change is to fully support group avatar management via IaC using the GitLab Terraform provider.