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

[peterfriese]

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