Add billable member counts to Group API entity

What does this MR do?

This MR adds a new billable_members_count attribute to Group API calls when the user is a group admin or instance admin.

This will enable our self managed customers to more easily view the count for their various groups (including sub-groups), which has been problematic in the past.

Refs #322815 (closed)

How to test

With either a instance or group admin user, perform the following curl:

curl --request GET \
  --url http://127.0.0.1:3000/api/v4/groups/<group id> \
  --header 'private-token: <your token>'

Does this MR meet the acceptance criteria?

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