Add Upload-Part-Number in the header during parallel uploads

so that the server can directly specify the PartNumber during multipart uploads, instead of first writing to the object and then using a copy method

```

          // Add the header to indicate the this is a partial upload.
          headers: {
            ...this.options.headers,
            'Upload-Concat': 'partial',
            'Upload-Part-Number': String(index + 1),
          },
```