Add FolderProvider

Toolkit.Foundation/SelectFoldersHandler.cs

@@ -0,0 +1,20 @@
+namespace Toolkit.Foundation;
+
+public class SelectFoldersHandler(IFolderProvider folderProvider) :
+    IHandler<SelectionEventArgs<FolderFilter>, IReadOnlyCollection<string>?>
+{
+    public async Task<IReadOnlyCollection<string>?> Handle(SelectionEventArgs<FolderFilter> args, 
+        CancellationToken cancellationToken)
+    {
+        if (args.Sender is FolderFilter filter)
+        {
+            if (await folderProvider.SelectFolders(filter)
+                is { Count: > 0 } folders)
+            {
+                return folders;
+            }
+        }
+
+        return default;
+    }
+}