Add new fields to GroupType

This issue is to resolve #409311 (comment 1496202978) to make the UI align with the designs in #406613 (closed)

We will need the following data that is not currently available in app/graphql/types/group_type.rb.

• descendantGroupsCount
• projectsCount
• groupMembersCount
• accessLevel - Should return Types::AccessLevelType

Availability & Testing

• no E2E tests needed
• Adding tests to the unit/integration level covering the new elements will be sufficient

Implementation guide

In app/controllers/groups/children_controller.rb, it uses GroupDescendantsFinder.new to setup the children. We could use similar logic to get the above counts

• with_selects_for_list to preload counts
  • descendantGroupsCount => is subgroup_count
  • projectsCount => is project_count
  • groupMembersCount => is member_count

See https://docs.gitlab.com/ee/development/graphql_guide/ for GraphQL help