Add a http.Client compatible client

[escamoteur]

With the introduction of the cupertino_http and cronet_http only Windows and Linux Dart and Flutter apps can't use the benefits of HTTP2 while staying compatible with he abstract http.Client interface.
So it would be nice if this was added to this package.

[brianquinlan]

Maybe it would make sense for someone to update package:flutter_curl (or create a new curl-based package) to support the package:http Client interface?

Or use package:rhttp.