Add Storage list APIs and progress observers

[IT-Guy007]

Implement Firebase Storage listing and byte-level progress observation: add async list/maxResults/pageToken and listAll on StorageReference, introduce StorageListResult wrapper (items, prefixes, pageToken), and add StorageTaskStatus/StorageProgress/StorageTaskSnapshot types. Implement observe/removeObserver/removeAllObservers for UploadTask and FileDownloadTask (with internal observer tracking) and provide no-op/overrides for other download tasks. Update tests to exercise list APIs and observer types, and update README to reflect increased Storage support; remove related TODO comments.

Thank you for contributing to the Skip project! Please use this space to describe your change and add any labels (bug, enhancement, documentation, etc.) to help categorize your contribution.

Please review the contribution guide at https://skip.dev/docs/contributing/ for advice and guidance on making high-quality PRs.

Skip Pull Request Checklist:

• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
• OPTIONAL: I have tested my change on an Android emulator or device

---

• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

---

Used for debugging observe functionality

[marcprux]

Looks good! It just needs one quick fix.

[marcprux]

Thanks for the contribution!