text: Title: Extend support for group-level service-accounts
  Body: In addition of [issue 2812](https://github.com/python-gitlab/python-gitlab/issues/2812) and [feature 2851](https://github.com/python-gitlab/python-gitlab/pull/2851) it would be nice to extend the interaction with group-level service-accounts.
  
  Currently only creation is supported, but it would be a nice feature to also support listing and deleting of service-accounts under a specific group.
  
  API documentation: 
  
  - https://docs.gitlab.com/ee/api/groups.html#list-service-account-users
  - https://docs.gitlab.com/ee/api/groups.html#list-service-account-users
  