[camera_avfoundation] Move camera_avfoundation to Darwin

[Mairramer]

This PR moves the camera_avfoundation plugin implementation to Darwin, aiming to consolidate the shared AVFoundation codebase for both iOS and macOS platforms.

Part of flutter/flutter#41708

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Footnotes

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[gemini-code-assist]

Code Review

This pull request moves the camera_avfoundation package source files to a shared Darwin directory. It updates the Pigeon Swift output path to the new darwin directory, adds sharedDarwinSource: true to the iOS plugin configuration in pubspec.yaml, and documents the change in the changelog. There are no review comments, and I have no feedback to provide.