CmdPal: Make it easier to add APIs in the future (#41056)

We learned a lot about adding interfaces in WinRT this week. I figured
I'd send a PR to write it all down.

src/modules/cmdpal/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/CommandProvider.cs

@@ -6,7 +6,7 @@ using Windows.Foundation;
 
 namespace Microsoft.CommandPalette.Extensions.Toolkit;
 
-public abstract partial class CommandProvider : ICommandProvider
+public abstract partial class CommandProvider : ICommandProvider, ICommandProvider2
 {
     public virtual string Id { get; protected set; } = string.Empty;
 
@@ -47,4 +47,26 @@ public abstract partial class CommandProvider : ICommandProvider
         {
         }
     }
+
+    /// <summary>
+    /// This is used to manually populate the WinRT type cache in CmdPal with
+    /// any interfaces that might not follow a straight linear path of requires.
+    ///
+    /// You don't need to call this as an extension author.
+    /// </summary>
+    /// <returns>an array of objects that implement all the leaf interfaces we support</returns>
+    public object[] GetApiExtensionStubs()
+    {
+        return [new SupportCommandsWithProperties()];
+    }
+
+    /// <summary>
+    /// A stub class which implements IExtendedAttributesProvider. Just marshalling this
+    /// across the ABI will be enough for CmdPal to store IExtendedAttributesProvider in
+    /// its type cache.
+    /// </summary>
+    private sealed partial class SupportCommandsWithProperties : IExtendedAttributesProvider
+    {
+        public IDictionary<string, object>? GetProperties() => null;
+    }
 }