Context: dotnet#16346

This addresses the memory leak discovered by:

    src/Core/src/Platform/iOS/MauiRefreshView.cs(17,10): error MA0002: Member '_refreshControlParent' could cause memory leaks in an NSObject subclass. Remove the member, store the value as a WeakReference, or add the [UnconditionalSuppressMessage("Memory", "MA0002")] attribute with a justification as to why the member will not leak.
    src/Core/src/Platform/iOS/MauiRefreshView.cs(18,11): error MA0002: Member '_contentView' could cause memory leaks in an NSObject subclass. Remove the member, store the value as a WeakReference, or add the [UnconditionalSuppressMessage("Memory", "MA0002")] attribute with a justification as to why the member will not leak.
    src/Core/src/Platform/iOS/MauiRefreshView.cs(19,20): error MA0002: Member '_refreshControl' could cause memory leaks in an NSObject subclass. Remove the member, store the value as a WeakReference, or add the [UnconditionalSuppressMessage("Memory", "MA0002")] attribute with a justification as to why the member will not leak.

I could reproduce a leak in a test such as:

    await InvokeOnMainThreadAsync(() =>
    {
        var layout = new Grid();
        var refreshView = new RefreshView();
        layout.Add(refreshView);
        var handler = CreateHandler<LayoutHandler>(layout);
        viewReference = new WeakReference(refreshView);
        handlerReference = new WeakReference(refreshView.Handler);
        platformViewReference = new WeakReference(refreshView.Handler.PlatformView);
    });

    await AssertionExtensions.WaitForGC(viewReference, handlerReference, platformViewReference);
    Assert.False(viewReference.IsAlive, "RefreshView should not be alive!");
    Assert.False(handlerReference.IsAlive, "Handler should not be alive!");
    Assert.False(platformViewReference.IsAlive, "PlatformView should not be alive!");

Solved the issues by making a non-NSObject `MauiRefreshViewProxy` class.