add sort option to files api

fixes CNVS-11674

test plan:
 - consult the documentation for the files api index action
 - test each sort option in both asc and desc order

Change-Id: Idedfb3ad61473567d92f97975c17adfd58160a65
Reviewed-on: https://gerrit.instructure.com/33601
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>