aws: support multipart copy for objects larger than 5GB

[james-rms]

Which issue does this PR close?

#563

Rationale for this change

Today, users that attempt to copy a >5GB object in S3 using object_store will see this error:

Server returned non-2xx status code: 400 Bad Request: 
<Error><Code>InvalidRequest</Code><Message>
The specified copy source is larger than the maximum allowable size for a copy source: 5368709120
</Message></Error>

The way to get around this problem per AWS's docs is to do the copy in several parts using multipart copies. This PR adds that functionality to the AWS client.

It adds two additional configuration parameters:

    /// The size threshold above which copy uses multipart copies under the hood. defaults to 5GB.
    multipart_copy_threshold: u64
    /// When using multipart copies, the part size used. Defaults to 5GB.
    multipart_copy_part_size: u64

The defaults are chosen to minimise surprise: if people are used to copies not requiring several requests, we don't switch to that method until it's absolutely necessary, and when necessary, we use as few parts as possible.

What changes are included in this PR?

See above.

Are there any user-facing changes?

Yes - these configuration parameters should be covered by the docstring changes.

TODO

• Will add tests after maintainers take a quick look at the expected behavior.

[tustvold]

I think this probably warrants a higher level ticket to discuss how we should support this, as a start it would be good to understand how other stores, i.e. GCS and Azure handle this, so that we can develop an abstraction that makes sense.

In particular I wonder if adding this functionality would make more sense as part of the multipart upload functionality? This of course depends on what other stores support.

In general filing an issue first to get consensus on an approach is a good idea before jumping in on an implementation

[james-rms]

Great, created #563.