Added Talk.initPushNotificationHandlers and Talk.handleFCMBackgroundMessage

[bugnano]

See https://talkjs.slack.com/archives/C02PH434FAT/p1733135037788469

This PR adds the registerBackgroundHandler option to the AndroidSettings that allows to not register the background handler (defaults to true).

It also adds the Talk.handleFCMBackgroundMessage function, that is to be called inside a background notification handler.

import 'package:flutter/material.dart';
import 'package:talkjs_flutter/talkjs_flutter.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_messaging/firebase_messaging.dart';
import 'firebase_options.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  if (Platform.isAndroid) {
    await Firebase.initializeApp(
      options: DefaultFirebaseOptions.currentPlatform,
    );
  }

  await Talk.registerPushNotificationHandlers(
    androidSettings: const AndroidSettings(
      channelId: 'com.example.talkjsflutter.messages',
      channelName: 'Messages',
      registerBackgroundHandler: false,
    ),
    iosSettings: const IOSSettings(useFirebase: false),
  );

  FirebaseMessaging.onBackgroundMessage(myBackgroundMessageHandler);

  runApp(const MyApp());
}

@pragma("vm:entry-point")
Future<void> myBackgroundMessageHandler(RemoteMessage firebaseMessage) async {
  // Call the TalkJS-specific push notification handler
  final notificationHandled = await Talk.handleFCMBackgroundMessage(firebaseMessage);

  if (!notificationHandled) {
    // Handle non-TalkJS originated push notification
  }
}

[vickz84259]

Cool. Everything checks out.

Sidenote: The diff view on Github for the changes in notification.dart was almost impossible to tell how things had changed. I ended up just copying the raw file into VSCode and comparing with the previous code to understand.

[vickz84259]

Just a minor suggestion. Feel free to merge and release afterwards.