Add ICameraProvider.AvailableCamerasChanged

[TheCodeTraveler]

Description of Change

This PR adds the event ICameraProvider.AvailableCamerasChanged.

When IReadOnlyList<CameraInfo>? AvailableCameras in ICamera changes, ICameraProvider.AvailableCamerasChanged will fire.

This is a helpful event that allows developers to refresh their UI every time a new camera is detected as demonstrated in CameraPageViewModel:

	public CameraViewViewModel(ICameraProvider cameraProvider)
	{
		this.cameraProvider = cameraProvider;

		cameraProvider.AvailableCamerasChanged += HandleAvailableCamerasChanged;
	}

	public IReadOnlyList<CameraInfo> Cameras => cameraProvider.AvailableCameras ?? [];

	void HandleAvailableCamerasChanged(object? sender, IReadOnlyList<CameraInfo>? e)
	{
		OnPropertyChanged(nameof(Cameras));
	}

PR Checklist

• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

This is a small quality-of-life improvement that I discovered when refactoring the #2634

[TheCodeTraveler]

Hey @bijington! What are your thoughts on whether we should add this to the docs?

I've added https://github.com/CommunityToolkit/Maui/issues?q=state%3Aopen%20label%3A%22pending%20documentation%22, but, when I went to open a Docs PR, I noticed that we don't have ICameraProvider documented which kinda makes sense since it's a lower-level API.

For the path forward, do you recommend we add Docs for ICameraProvider? Or is it cool to push this PR through without docs?