md5 reported in list_path is wrong for files on S3 uploaded as multipart

[kmichel-aiven]

What happened?

rohmu assumes the Etag reported by S3 listObjectV2 is an MD5, this is false if the
file was uploaded as multipart, or for some cases of server side encryption:

https://docs.aws.amazon.com/AmazonS3/latest/API/API_Object.html#AmazonS3-Type-Object-ETag

https://github.com/aiven/rohmu/blob/8ae24adf491f26b676ed1ae0339aaa2dd87d96ae/rohmu/object_storage/s3.py#L248

[giacomo-alzetta-aiven]

I believe @fingon knew this very well when adding this. He chose to use md5 for "hash" because it was already used by other backends. The local backend reports the sha256 hash in the md5 field as an other example...
And swift is the only one using a generic hash key instead of md5 (which I'm not sure how it is computed).

We could probably review this handling..