Add member counts to GitLab.com billing areas [BE]

Problem to solve

We'll need backend support to make https://gitlab.com/gitlab-org/gitlab-ee/issues/6898 possible.

Proposal

We'll need to support:

• Obtaining the # of members paid for in a GitLab.com group (similar to "users in license" for self-managed).
• Obtaining the unique active members in a GitLab.com group (and all subgroups).
  • We want to be able to display member count, and a member list similar to the Users table for self-managed.
  • We also want to include last_activity_on.

What does success look like, and how can we measure that?

(If no way to measure success, link to an issue that will implement a way to measure this)

Links / references