CmdPal: Once again, I am asking you to fix form submits (#41010)

Closes #40979

Usually, you're supposed to try to cast the action to a specific
type, and use those objects to get the data you need.
However, there's something weird with AdaptiveCards and the way it
works when we consume it when built in Release, with AOT (and
trimming) enabled. Any sort of `action.As<IAdaptiveSubmitAction>()`
or similar will throw a System.InvalidCastException.

Instead we have this horror show.

The `action.ToJson()` blob ACTUALLY CONTAINS THE `type` field, which
we can use to determine what kind of action it is. Then we can parse
the JSON manually based on the type.

src/modules/cmdpal/ext/SamplePagesExtension/Pages/SampleContentPage.cs

@@ -225,6 +225,11 @@ internal sealed partial class SampleContentForm : FormContent
                     }
                 ]
             }
+        },
+        {
+            "type": "Action.OpenUrl",
+            "title": "Action.OpenUrl",
+            "url": "https://adaptivecards.microsoft.com/"
         }
     ]
 }