[request]: Pre-signed URLs for Cloudfront

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I expected https://github.com/awslabs/aws-sdk-rust/issues/139 would implement pre-signed url's for CloudFront (as the description of the request states it will include that), but I can not find how I should sign URL's in CloudFront. I suspect that feature didn't make it.

Start by reviewing the linked issue 139 and the current aws-sdk-rust CloudFront support to determine whether URL signing is exposed anywhere. Done means CloudFront pre-signed URLs are available through the SDK with usage documented; no specific file or test is named in this issue.

Written by the indexing model from the issue text.