[GHF] Add ability to specify team as the approver
All GitHub teams belong to organization, and therefore must be
referenced as `org/team_name`
Add memoizable `gh_get_team_members` and tests that it can be used to
query existing and non-existing teams
Note: Requires "org.read" permission on the token to work function
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74892
Approved by: https://github.com/janeyx99
