[Windows] Add support for custom callback schemes

[Mino5531]

This PR adds supports for custom callback schemes during authentication on Windows, by opening a new instance of the application that displays a webview (Microsoft WebView2).

Potential issues?

• Microsoft WebView2 must be installed or packaged with the app (preinstalled as part of Windows 11 and on some Windows 10 devices but not all) Read more
• App developers must add the following code snippet to their main function to block the webview window from executing code twice

  void main(List<String> args) {
  //Add this
  if (runWebViewTitleBarWidget(args)) {
  return;
  }
  //---
  runApp(const MyApp());
  }

• Will break the old way of using this library (using http://localhost as the scheme) as there will no longer be a local server that listens for calls

fixes #25 And should also (hopefully) fix #73

[ThexXTURBOXx]

Thank you very much for your PR! I will review it probably during the weekend :) The release will probably be version 4.0.0 then due to the breaking changes, but let's see

[ThexXTURBOXx]

Sorry for the late response. I have merged your changes and made sure that the "legacy" way and your new approach are both still callable. I have also documented everything appropriately and will release 4.0.0-alpha.0 now. Thanks for contributing!