Support single-request multipart upload for setting metadata

[sumeetattree]

@ThouCheese Hi could you take a look at this PR and suggest any edits etc? This resolves this issue .

I've added tests and verified that the tests are passing for both the async and the sync version. Thanks!

[Elykz]

Maybe it would make sense to provide a similar interface to the create_streamed function, by adding a create_streamed_with method ?

[sumeetattree]

Yep, that's a good idea! Though I am not sure if I'll be able to quickly get back to it just yet. There has been some reprioritization at work and it may take a while for me to make edits. I'll try, but if someone else has immediate need for it please go ahead..

Another thought is to at least get this in and open another PR to support the streamed api.