Failing test for multipart uploads with checksums

[avantgardnerio]

Which issue does this PR close?

Closes #6793.

Rationale for this change

Described in issue

What changes are included in this PR?

A failing test

Are there any user-facing changes?

Multipart uploads fail

[thinkharderdev]

So there are a couple of issues with how multi-part upload is implemented:

1. The initial PUT request to CreateMultipartUpload must specify the checksum algo or else calling PutPart with the checksum will fail (eg set x-amz-checksum-algorithm: SHA256 on the request)
2. When calling CompleteMultipartUpload we need to send all the part checksums in the request (see `https://docs.aws.amazon.com/AmazonS3/latest/API/API_CompleteMultipartUpload.html)

So this means that we need to keep the checksums in the PartId for when we complete the upload

[tustvold]

I'm not sure about this, an AWS specific detail now leaks into the public trait for all stores... It also is not immediately obvious when the checksums are needed or not...

I think this needs a bit more thought, I'll try to find some time in the next few weeks

[avantgardnerio]

this, an AWS specific detail now leaks into the public trait for all stores... It also is not immediately obvious when the checksums are needed or not...

FWIW: I verified in the new tests we are now writing the checksums at the correct times for when the digest functionality is enabled.

Now I am just trying to get the original tests working for when it isn't.