Amazon S3 sync/check

Under the tools page, it would be useful for folks using WP Offload media and/or other S3 plugins to have a way to check that all their media was synced. This would not be for bulk-uploading media libraries, just to do a check on filesize. Basically comparing the S3 version to the local copy to make sure we sent the optimized version (and WebP derivatives) to S3.