Upload starts the process of a resumable upload with a cancellable context. It retries using the provided back off strategy until cancelled or the strategy indicates to stop retrying. It is called from the auto-generated API code and is not visible to the user. Before sending an HTTP request, Upload calls any registered hook functions, and calls the returned functions after the request returns (see send.go). rx is private to the auto-generated API code. Exactly one of resp or err will be nil. If resp is non-nil, the caller must call resp.Body.Close.