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.

functions/package.json

@@ -14,10 +14,11 @@
   "main": "index.js",
   "dependencies": {
     "firebase-admin": "^13.6.0",
-    "firebase-functions": "^7.0.0"
+    "firebase-functions": "^7.0.0",
+    "nodemailer": "^6.10.1"
   },
   "devDependencies": {
     "firebase-functions-test": "^3.4.1"
   },
   "private": true
-}
+}