Query 13F institutional holdings by filer CIK or by held ticker. Provide exactly one of `filer_cik` or `ticker`. When no `report_period` filter is supplied, `?filer_cik=...` returns the filer's most recent 13F and `?ticker=...` returns one position per institutional filer whose most recent 13F currently includes the ticker (filers who have since dropped the position are excluded).