Returns each sub-agency with its obligations, transaction count, and
new-award count for the given fiscal year. Useful for finding the
canonical subtier name to pass into search_awards() awarding_subagency.

sort accepts name, total_obligations, transaction_count, or
new_award_count (this endpoint has no outlay column; a former
'total_outlays' option was rejected by the API with HTTP 400).

fiscal_year defaults to the last completed fiscal year, not the current
one (USAspending's own default, which right after October 1 holds only a
few days of data). The answer always carries fiscal_year.