Add AsyncStream support for Cloud Firestore

[peterfriese]

Add AsyncSequence support for Firestore

This change introduces support for AsyncSequence to DocumentReference and Query, allowing developers to use for await in to receive real-time updates from Firestore.

The implementation wraps the existing snapshot listener APIs in an AsyncThrowingStream, providing a modern, Swift-native way to work with real-time data.

This change also includes unit tests for the new functionality.