cmdpal: A sample for tracking load/unload for pages (#39265)

Committing this sample for posterity's sake.

This is the product of an experiment: "can an extension know when it's
page was opened / closed?"

And without changing the actual SDK, this **is** possible. It relies on
the fact that CmdPal (the host) needs to register for the `ItemsChanged`
event on list pages, and it does that when the page is loaded, and it
unregisters itself when the page is closed.

This does require manually implementing `IListPage` - you can't just
`override` the `event` in the base class ([that's
illegal](https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1070)),
and using `new` to shadow it doesn't work either (probably for cswinrt
reasons).

This is the best I came up with.

src/modules/cmdpal/ext/SamplePagesExtension/SamplesListPage.cs

@@ -32,6 +32,11 @@ public partial class SamplesListPage : ListPage
             Title = "Dynamic List Page Command",
             Subtitle = "Changes the list of items in response to the typed query",
         },
+        new ListItem(new OnLoadPage())
+        {
+            Title = "Demo of OnLoad/OnUnload",
+            Subtitle = "Changes the list of items every time the page is opened / closed",
+        },
 
         // Content pages
         new ListItem(new SampleContentPage())