Added Real-Time Remote Config Update Support

[fhasse95]

This PR adds real-time Remote Config update support to SkipFirebaseRemoteConfig. This is achieved by a new addOnConfigUpdateListener API, which registers the native Firebase Remote Config update listener on Android and returns a ConfigUpdateListenerRegistration for removing it.

For more information, check out the updated README file.

Additionally, I made the package compatible with iOS 16 and macOS 13 again, since my app requires this and to keep it consistent with the rest of the Skip packages.

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

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• 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
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• 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.

The initial implementation was generated with assistance from Codex. The resulting code was manually reviewed, refined where necessary, and validated using my own app on iOS, iPadOS, macOS (Catalyst), and Android to ensure correctness and consistent cross-platform behavior.

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