[Draft][flutter_appauth][flutter_appauth_platform_interface] Add platform method to process callbacks which where not received nativly

[Xylez01]

Draft implementation for suggestion two mentioned in #358

Currently only did a quick version of the Obj-C impl.

The way this would work:

1. Authentication process is started using authorizeAndExchangeCode
2. The sign in callback is potentially caught by another plugin and handeld in the flutter app
3. In order to continue with the OAuth flow, the app sends the callback url to this plugin using processCallback