Add a `top-level-only` option for group graphql query

The following discussion from !171359 (merged) should be addressed:

• @arfedoro started a discussion: (+1 comment)

  question (non-blocking) ❓

  Do I understand right that, you use api instead of graphql, only for the sake of top_level_only option?

  If yes and, as general direction, we are steering away from using an api, what do you think about creating a follow-up to update graphql in order to include top_level_only option?