Implement bin/aws-s3 get ... and a basic S3 client API

Summary: Ref T5155. This implements pulling file data off S3 using a first-party, Signature v4-compatible API.

Test Plan: {F1057744}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5155

Differential Revision: https://secure.phabricator.com/D14979