[ios] fix memory leak in RefreshView

[jonathanpeppers]

Context: #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.