docs: Use Swift for notification service extension

[Gustl22]

Description

These docs provide a way to write a notification service extension in Swift instead of Objective-C

Checklist

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• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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[Gustl22]

The code block format was taken from (as ref):

flutterfire/docs/auth/federated-auth.md

Line 56 in 965260d

* {Web}

I don't know how this page looks rendered though.

[Gustl22]

The docs for iOS also describe a similar approach. It should maybe also be rewritten to work with Swift:
https://firebase.google.com/docs/cloud-messaging/ios/send-image

These docs aren't available publicly in this organization.