Add support for temporary files to file.allocate

Summary:
Ref T7148. I can do most of the export stuff by only modifying the Instances codebase, but want to upload all the backups and exports as temporary files and can't currently do this via the API.

Make the necessary API changes so that the export workflow can use them when it gets built out.

Test Plan: See next diff. Uploaded files with arc upload --temporary and saw them upload as temporary files.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7148

Differential Revision: https://secure.phabricator.com/D14055