Support Workhorse directly uploading files to S3

This supports the AWS S3 client that will be used by Workhorse in gitlab-workhorse!466 (merged).

This makes it possible to use S3 buckets with default KMS encryption and proper MD5 checksums.

Related issues:

1. gitlab-workhorse#222 (closed)
2. gitlab-workhorse#185 (closed)
3. gitlab-workhorse#210 (closed)