Support Content-MD5 header for checking object integrity when uploading object

See: https://docs.aws.amazon.com/AmazonS3/latest/userguide/checking-object-integrity.html#checking-object-integrity-md5

Client can specify the "Content-MD5" header to verify the integrity of the object as part of the upload process.

If the Content-MD5 does not match with the calculated md5, should throw BadDigest.

See: https://docs.aws.amazon.com/AmazonS3/latest/API/ErrorResponses.html

Since S3G already supports ETag which uses md5 hash of the object content, we can reuse it.

Note: If there is a BadDigest, we might need to clean up the data already written.