[ios/catalyst] fix memory leaks in `*Cell`

[jonathanpeppers]

Fixes: https://github.com/dotnet/maui/issues/20195 Context: https://github.com/AdamEssenmacher/MemoryToolkit.Maui/tree/main/samples

Retesting this sample with the ListView leak fixed, I noticed the sample reports 0 leaks! Unfortunately, it still displayed memory growing over time...

Taking a .gcdump snapshot, I noticed a cycle:

• ViewCellRenderer.ViewTableCell -> ViewCell _viewCell -> ViewCellRenderer -> ViewTableCell

I was able to write a test that reproduces the leak, and I extended it for every type of *Cell like TextCell, ImageCell, SwitchCell, etc.

The fixes are:

• ViewTableCell now uses a WeakReference<ViewCell> _viewCell

• CellTableViewCell now uses a WeakReference<Cell> _cell

• CellTableViewCell now uses a WeakEventManager for PropertyChanged, as the *Renderer subscribes to this event.

Note that I changed PropertyChanged to an event, which is an API change. (I can revisit this if needed)

[jsuarezruiz]

Needs a rebase to get the updated snapshot for Issue18242Test and fix the build.