[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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[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.

[Mairramer]

Maybe we should move the tests as well?

[stuartmorgan-g]

Maybe we should move the tests as well?

Yes, native unit tests that would be shared should move to darwin/Tests/, and the Xcode project updated to reference the files there, as with other plugins in the repo that use shared source.

[Mairramer]

Maybe we should move the tests as well?

Yes, native unit tests that would be shared should move to darwin/Tests/, and the Xcode project updated to reference the files there, as with other plugins in the repo that use shared source.

Done.

[stuartmorgan-g]

LGTM

[stuartmorgan-g]

Version/CHANGELOG override: The only failure mode for clients here should be a complete failure to build if it were incorrect, which CI would catch, so we can skip releasing this.

[okorohelijah]

from triage: is this still on your radar @LouiseHsu