Exposing the multipart-upload api to users

[johanot]

list_multiparts_uploads and abort_upload are already public functions.

But for client that support client-side-chunking, I currently have to assemble all chunks in a my application, only have them later re-chunked by rust-s3.

It should be possible for clients to make use of multipart upload directly, passing the relevant multipart state for each chunk uploaded.

Still draft: because docs (and possibly tests?) are missing.

[durch]

@johanot Thanks for the PR, I'd be happy to merge it in once its ready :)

[johanot]

Rebased this with master and revived the code + undrafted the PR. I've use this patch myself for some time and I think it's sad that I need a fork for this.

@durch Please let me know what I need to do to get this merged eventually, thanks :)

[durch]

@johanot Could you take a look at the conflict, I'd still like to get this merged, it slipped my mind, apologies for that

[johanot]

@durch Done!

[durch]

@johanot Amazing, thank you :)