Add bin/files cat to print a file to stdout

Summary:
Ref T7149. This makes debugging some of this stuff a bit easier by removing the HTTP part in the middle.

Particularly, I anticipate having this stream data chunk-by-chunk in the near future.

Test Plan: Ran files cat F23, got output.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12062