[ios] fix memory leak in CollectionView cells

[jonathanpeppers]

Context: #14664
Context: https://github.com/nacompllo/MemoryLeakEverywhere/tree/bugfix/memoryLeakItemsSource

In reviewing our latest changes in main with the above customer sample, I found that there appeared to be leaks related to MAUI's UITableViewCell subclasses when using CollectionView.

I was able to reproduce the issue in a test, such as:

// DataTemplate saves WeakReference to the View in a list
collectionView.ItemTemplate = new DataTemplate(() =>
{
    var label = new Label();
    labels.Add(new(label));
    return label;
});

// Create a cell and bind it to the template:
cell = new VerticalCell(CGRect.Empty);
cell.Bind(collectionView.ItemTemplate, bindingContext, collectionView);

// Check we have no leaks
foreach (var reference in labels)
{
    Assert.False(reference.IsAlive, "View should not be alive!");
}

After isolating the issue, I found the issue was the TemplatedCell.PlatformHandler property:

internal IPlatformViewHandler PlatformHandler { get; private set; }

This stores a copy of the LabelHandler in our test/example.

The problem with UITableViewCell is that UIKit holds onto these and reuses them. This means that UIKit may keep the LabelHandler alive longer than needed.

It also appears to be a somewhat complex circular reference:

• CollectionView -> handlers / etc. -> TemplatedCell -> LabelHandler -> Label -> CollectionView

I made the PlatformHandler use a WeakReference as its backing field and the problem goes away!

I will retest #14664 to verify if it is fully solved.