Add a `PhutilAWSS3Future` class

Summary: Ref T5155. This doesn't do anything currently, but should allow us to perform S3 operations without the [[http://undesigned.org.za/2007/10/22/amazon-s3-php-class/ | 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