Introduces a new `configUpdates` property to `RemoteConfig`, providing an `AsyncSequence` for consuming real-time configuration updates.
This modern, Swift Concurrency-native API offers a cleaner alternative to the existing closure-based listener, allowing developers to handle config updates using `for await` loops.
Key changes:
- Added `RemoteConfig.configUpdates` returning `RemoteConfigUpdateSequence`.
- Implemented `RemoteConfigUpdateSequence` with a built-in iterator that manages the lifecycle of the underlying listener.
- Aligned implementation with the recently merged Firestore AsyncSequence patterns.
- Comprehensive unit tests verified for listener registration, event yielding, and graceful termination.
