Migrate the "SessionManager" from "ChangeNotifier" to broadcast stream for auth state changes

[dkbast]

It would be great if we would get an instance of the current state when the on change callback is invoked. Imho stream subscriptions are a bit easier to manage than change notifier listeners. With the change notifier you always need to pass in an instance of the notifier into the "addListener" which then is no longer a "pure function". Also removing listeners is imho much more cumbersome than cancelling stream subscriptions.

https://github.com/serverpod/serverpod/blob/main/modules/serverpod_auth/serverpod_auth_shared_flutter/lib/src/session_manager.dart