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/repositories/auth_repository.dart

@@ -312,6 +312,7 @@ class AuthRepository {
       if (token != null) {
         await _firestore.collection('users').doc(userId).set({
           'fcmToken': token,
+          'fcmTokens': FieldValue.arrayUnion([token]),
         }, SetOptions(merge: true));
       }
     } catch (e, stackTrace) {