Add a PhutilAWSS3Future class

Summary: Ref T5155. This doesn't do anything currently, but should allow us to perform S3 operations without the Amazon S3 PHP Class.

Test Plan: I'm not really sure how to test this... if you have any ideas then please let me know.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5155

Differential Revision: https://secure.phabricator.com/D9781