aws: support multipart copy for objects larger than 5GB

[james-rms]

Which issue does this PR close?

• close Enable AWS client to copy objects >5GB in size #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.

[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.

[james-rms]

@tustvold updated to pass CI and a small tweak to avoid overflow.

[alamb]

Thanks @james-rms and @tustvold -- the high level idea seems reasonable to me, but I think this code needs tests (maybe unit tests?) or something otherwise we may break the functionality inadvertently in some future refactor

[james-rms]

@tustvold I've refactored slightly for unit tests and added a couple of integration tests as well. Please take another look when you have some time.