Make "arc download" use "file.search" if available
Summary: Fixes T8348. Just use normal HTTP GET to download files if the server is sufficiently modern.
Test Plan: Downloaded various files with --as, --show, large files, small files, old server, new server.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T8348
Differential Revision: https://secure.phabricator.com/D17614