Improvements for ee/lib/ee/api/members.rb

The following discussions from !48368 (merged) should be addressed:

• @tkuah started a discussion: (+2 comments)

  This adds 200+ methods to the public instance methods of Group. Should we move the methods we want into a different concern that we can share amongst SortingHelper, Group, and MembersHelpers ?

  On a related note, the #billed_users_for method below probably belongs outside of the Group model, maybe it belongs in MembersHelpers ?

• @tkuah started a discussion:

  Consider extracting this method into its own Finder, but this can be done in a follow-up

• @tkuah started a discussion:

  Maybe we can make order_by a kwarg ?

  def billed_users_for(group, search_term, order_by: 'name_asc')

• @tkuah started a discussion:

  I don't see a spec that tests a user without this ability is rejected. Could we please add that test ?

  https://gitlab.com/gitlab-org/gitlab/-/blob/778f2456fe65caf6008027a2058beb10e10374c1/ee/spec/requests/api/members_spec.rb#L334

/cc @ebaque