feat(remote_config): Migrate firebase_remote_config iOS and Android to Pigeon

[cynthiajoan]

This commit migrates the native communication layer for the firebase_remote_config plugin on iOS and Android from MethodChannel to Pigeon.

Work Done:

1. Defined Pigeon API: Created messages.pigeon in the platform interface package, defining the Host API (FirebaseRemoteConfigHostApi) and data structures based on the existing MethodChannel methods.
2. Configured Pigeon Generation: Updated messages.pigeon with @ConfigurePigeon to specify output paths for Dart, Swift, and Kotlin in the main firebase_remote_config package. (Note: Actual generation was not performed due to tool limitations).
3. Implemented iOS Pigeon Host API: Refactored FLTFirebaseRemoteConfigPlugin.m (Objective-C) to implement the FirebaseRemoteConfigHostApi protocol generated by Pigeon. Replaced MethodChannel handling with Pigeon API calls, adapting helper methods and updating plugin registration. EventChannel logic for onConfigUpdated remains.
4. Implemented Android Pigeon Host API: Refactored FirebaseRemoteConfigPlugin.java to implement the FirebaseRemoteConfigHostApi interface generated by Pigeon. Replaced MethodChannel handling, adapted helper methods, converted data types, and updated plugin registration to use Pigeon. EventChannel logic remains.

Next Steps (Incomplete):

The Dart side implementation is pending:

• Update Dart Platform Interface: Create a new Dart class (PigeonFirebaseRemoteConfig) extending FirebaseRemoteConfigPlatform in the firebase_remote_config_platform_interface package. This class should use the generated Dart Pigeon client (messages.g.dart) to call the native Host APIs.
• Update Main Plugin Dart Code: Update firebase_remote_config's main Dart code to use the new PigeonFirebaseRemoteConfig implementation.
• Remove MethodChannel Code: Delete the old MethodChannel implementation files in the platform interface package.
• Testing: Run existing tests and potentially add new ones for Pigeon integration.

This includes the completed native iOS and Android migrations. Further work is needed on the Dart side to fully utilize the Pigeon implementation.

Description

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