Add support for getting the progress of a file upload / download

Hi,

When a user uploads a file in my app, I want to show him a progress bar. I'm looking for a way to listen to the amount of bytes sent in MultipartRequest, but I think it's not supported. Apparently you can do this with dart:io (https://stackoverflow.com/questions/50455131/flutter-how-to-get-upload-download-progress-for-http-requests?rq=1, https://medium.com/flutter-community/how-to-show-download-progress-in-a-flutter-app-8810e294acbd).

Can you add a support for this? Thanks.

Start with MultipartRequest and the dart:io upload and download approaches linked in the issue. No repository file or test is named; done means callers can observe transfer progress for file uploads and downloads so they can drive a progress bar.

Written by the indexing model from the issue text.