Add Flutter and/or Dart runtime version to Ably-Agent header, if possible

[QuintinWillison]

The work done under #188 only added the {{ably-flutter}} product.

It looks like {{dart:io}}'s {{Platform}} class has a [static version property|https://api.flutter.dev/flutter/dart-io/Platform/version.html].

In terms of Flutter version, [this approach|https://stackoverflow.com/a/60581960/392847] seems very hacky and would require the app builder to do something more than simply include our SDK. As such, it feels like a non-starter for our context.