Add FileUpload

* adds uploads.stripe.com endpoint
* adds support for multipart content
* some interface changes since we now have multiple backends
* bump to 4.0.0 since this is breaking

r? @wangjohn 
cc @metcalf (since it changes some of the interfaces you're relying on)
`BackendConfiguration` feels like a bit of a misnomer -- it isn't just configuration but the full behavior.