CmdPal: Filters for DynamicListPage? Yes, please. (#40783)

Closes: #40382

## To-do list

- [x] Add support for "single-select" filters to DynamicListPage
- [x] Filters can contain icons
- [x] Filter list can contain separators
- [x] Update Windows Services built-in extension to support filtering by
all, started, stopped, and pending services
- [x] Update SampleExtension dynamic list sample to filter.

## Example of filters in use

```C#
internal sealed partial class ServicesListPage : DynamicListPage
{
    public ServicesListPage()
    {
        Icon = Icons.ServicesIcon;
        Name = "Windows Services";

        var filters = new ServiceFilters();
        filters.PropChanged += Filters_PropChanged;
        Filters = filters;
    }

    private void Filters_PropChanged(object sender, IPropChangedEventArgs args) => RaiseItemsChanged();

    public override void UpdateSearchText(string oldSearch, string newSearch) => RaiseItemsChanged();

    public override IListItem[] GetItems()
    {
       // ServiceHelper.Search knows how to filter based on the CurrentFilterIds provided
        var items = ServiceHelper.Search(SearchText, Filters.CurrentFilterIds).ToArray();

        return items;
    }
}

public partial class ServiceFilters : Filters
{
    public ServiceFilters()
    {
        // This would be a default selection. Not providing this will cause the filter
        // control to display the "Filter" placeholder text.
        CurrentFilterIds = ["all"];
    }

    public override IFilterItem[] GetFilters()
    {
        return [
            new Filter() { Id = "all", Name = "All Services" },
            new Separator(),
            new Filter() { Id = "running", Name = "Running", Icon = Icons.GreenCircleIcon },
            new Filter() { Id = "stopped", Name = "Stopped", Icon = Icons.RedCircleIcon },
            new Filter() { Id = "paused", Name = "Paused", Icon = Icons.PauseIcon },
        ];
    }
}
```

## Current example of behavior


https://github.com/user-attachments/assets/2e325763-ad3a-4445-bbe2-a840df08d0b3

---------

Co-authored-by: Mike Griese <migrie@microsoft.com>

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

@@ -0,0 +1,23 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace Microsoft.CommandPalette.Extensions.Toolkit;
+
+public abstract partial class Filters : BaseObservable, IFilters
+{
+    public string CurrentFilterId
+    {
+        get => field;
+        set
+        {
+            field = value;
+            OnPropertyChanged(nameof(CurrentFilterId));
+        }
+    }
+
+    = string.Empty;
+
+    // This method should be overridden in derived classes to provide the actual filters.
+    public abstract IFilterItem[] GetFilters();
+}

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

@@ -4,6 +4,6 @@
 
 namespace Microsoft.CommandPalette.Extensions.Toolkit;
 
-public partial class SeparatorContextItem : ISeparatorContextItem
+public partial class Separator : ISeparatorContextItem, ISeparatorFilterItem
 {
 }

src/modules/cmdpal/extensionsdk/Microsoft.CommandPalette.Extensions/Microsoft.CommandPalette.Extensions.idl

@@ -122,7 +122,7 @@ namespace Microsoft.CommandPalette.Extensions
     interface ISeparatorFilterItem requires IFilterItem {}
     
     [contract(Microsoft.CommandPalette.Extensions.ExtensionsContract, 1)]
-    interface IFilter requires IFilterItem {
+    interface IFilter requires INotifyPropChanged, IFilterItem {
         String Id { get; };
         String Name { get; };
         IIconInfo Icon { get; };
@@ -131,7 +131,7 @@ namespace Microsoft.CommandPalette.Extensions
     [contract(Microsoft.CommandPalette.Extensions.ExtensionsContract, 1)]
     interface IFilters {
         String CurrentFilterId { get; set; };
-        IFilterItem[] Filters();
+        IFilterItem[] GetFilters();
     }
     
     struct Color