[FR]: Implement Swift AsyncSequence APIs for real-time streams

### Description

This epic tracks the effort to introduce modern, AsyncSequence-based APIs for all real-time event streams in the Firebase Apple SDK. The goal is to provide a more idiomatic, developer-friendly way to consume Firebase data streams, aligning with Swift's structured concurrency model.

## Design document
(provide link to design document once its added to the repo)

## Implementation checklist

This epic will be completed by implementing and merging separate PRs for each of the following Firebase products. Each PR should link back to this issue.

- [ ] Realtime Database
- [ ] Cloud Storage
- [ ] Cloud Messaging

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This issue will serve as the central hub for tracking the progress of this initiative. Once all the tasks in the checklist are complete, this epic can be closed.

### API Proposal

_No response_

### Firebase Product(s)

Authentication, Database, Firestore, Storage, Remote Config, Messaging