deploy: Set an MD5 attribute and use that if eTag not available

deploy: Set an MD5 attribute and use that if eTag not available

During Hugo deploy when a remote MD5 is invalid (e.g due to multipart eTag) Hugo reads the entire remote file and calculates the MD5 again which can be slow.

This commit updates the file upload so that it will also store an MD5 hash in the cloud provider's attributes. e.g in AWS this looks like x-amz-meta-md5chksum: 26fe392386a8123bf8956a16e08cb841.