feat: Add flutterfire_messenger package

[naipaka]

🔗 Issue Link

closes #2

🙌 What I did

• Add flutterfire_messenger package

✍️ What I didn't do

• Introduce other notification-related packages such as flutter_local_notifications
  • I think it's fine to add them in the future as needed
• Create an example package for altfire_messenger #26

✅ Verification

• Android
• iOS
• macOS
• Web

Screenshots

Additional Information

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d23f430) 53.99% compared to head (e2b38f5) 54.47%.

Files	Patch %	Lines
...kages/flutterfire_messenger/lib/src/messenger.dart	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   53.99%   54.47%   +0.48%     
==========================================
  Files          10       11       +1     
  Lines         263      268       +5     
==========================================
+ Hits          142      146       +4     
- Misses        121      122       +1     

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[riscait]

/// flutterfire_messenger with FlutterFire Messaging, Local Notifications,
/// In-App Messaging, and Firebase Installation ID.

It says "firebase_in_app_messaging", so it would be better to add firebase_in_app_messaging to pubspec's dependencies. (since I think just adding the package would give us the basic functionality of FIAM)

[naipaka]

It says "firebase_in_app_messaging", so it would be better to add firebase_in_app_messaging to pubspec's dependencies. (since I think just adding the package would give us the basic functionality of FIAM)

Thank you for your review! Indeed, it seems like a good idea to add the firebase_in_app_messaging package. I will add it!