WIP feat(messaging): add support for Android Config V2

[jonathanedey]

• Add androidV2 property to BaseMessage and MulticastMessage interfaces
• Introduce AndroidConfigV2, AndroidRemoteNotificationConfig, and AndroidBackgroundSyncConfig types
• Implement validation for androidV2 properties, including nested remoteNotification and backgroundSync exclusivity constraints
• Enforce mutual exclusivity between legacy android and new androidV2 payload configurations
• Add unit tests validating androidV2 property serialization, types, and error constraints
• Export new V2 interfaces

[gemini-code-assist]

Code Review

This pull request introduces the Android V2 configuration (androidV2) for Firebase Cloud Messaging (FCM) payloads, supporting remote notifications and background sync. It includes new TypeScript interfaces, validation logic, and comprehensive unit tests. The feedback suggests refactoring the validateAndroidConfigV2 function to use the existing renameProperties utility for renaming remoteNotification and backgroundSync properties, which would improve consistency and simplify the code.