Improve temporary file upload API and add viewPolicy support

Summary: Ref T7148. In D14056, I let arc upload upload temporary files, but this is a better way to do some of the internals. Also add support for setting a viewPolicy.

Test Plan: Used arc upload, arc upload --temporary.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7148

Differential Revision: https://secure.phabricator.com/D14075