Support creating signed URLs

[clue]

Right now, this library focuses on building a filesystem-like API (#1). On top of this, we should also provide a way to create signed URLs for private S3 objects so that they can be accessed directly through a standard HTTP GET request.

See also https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/private-content-signed-urls.html