Ensure Content-MD5 header is included on AWS backups

What does this MR do and why?

Unlike the AWS CLI, the fog-aws gem only includes the Content-MD5 HTTP header in a PutObject call if multipart uploads are used. However, this prevents Amazon Object Lock from working because Object Lock requires some checksum to be available.

https://github.com/fog/fog-aws/issues/750 is an upstream issue to include this header by default, but to unblock backups we add it manually.

References

Relates to #335775

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