feat: Implement trip invitation functionality and notification handling

- Added TripInvitationRepository for managing trip invitations.
- Created TripInvitation model with serialization methods.
- Implemented notification payload parser for handling FCM notifications.
- Enhanced NotificationService to manage trip invitations and related actions.
- Updated UserRepository to include user search functionality.
- Modified AuthRepository to store multiple FCM tokens.
- Added tests for trip invitation logic and notification payload parsing.
- Updated pubspec.yaml and pubspec.lock for dependency management.

lib/blocs/user/user_bloc.dart

@@ -92,6 +92,7 @@ class UserBloc extends Bloc<event.UserEvent, state.UserState> {
           LoggerService.info('UserBloc - Updating FCM token in Firestore');
           await _firestore.collection('users').doc(currentUser.uid).set({
             'fcmToken': fcmToken,
+            'fcmTokens': FieldValue.arrayUnion([fcmToken]),
           }, SetOptions(merge: true));
           LoggerService.info('UserBloc - FCM token updated');
         } else {