[flutterfire_ui] Export auth_state and auth_flow

[porfirioribeiro]

I'm trying to add an action to act when a social login fails because it's already assigned to other social provider, but the DifferentSignInMethodsFound is not exported
My solution was to import from src

import 'package:flutterfire_ui/src/auth/auth_state.dart';
import 'package:flutterfire_ui/src/auth/auth_flow.dart';

final actions=[
  AuthStateChangeAction<AuthFailed>((context, state) {
    if (state.exception is AuthCancelledException) {
      ScaffoldMessenger.of(context).showSnackBar(SnackBar(
        content: Text(translate('login.socialLoginCanceled')),
      ));
    }
  }),
  AuthStateChangeAction<DifferentSignInMethodsFound>((context, state) {
    ScaffoldMessenger.of(context).showSnackBar(SnackBar(
      content: Text(translate('login.socialLoginConflict')),
    ));
  })
];