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