Starts a new multipart upload.
This method starts a new multipart upload for an object named ObjectName, in the bucket currently selected by Bucket, and returns the upload Id that the server associates with it. This upload Id can then be used to call the following methods:
If there are any metadata items present in the Metadata collection, they will be included in the creation request, and will be applied to the final object after the multipart upload is completed with CompleteMultipartUpload.
Multipart uploads never expire, they must be explicitly completed or aborted using CompleteMultipartUpload or AbortMultipartUpload. The ListMultipartUploads method can be used to retrieve a list of current multipart uploads.
Note: This method is not supported when ServiceProvider is spGoogleStorage (2); Google Cloud Storage does not support multipart uploads.