Start to port main gut of the foundation

Toolkit.Foundation/Configurations/WriteHandler.cs

@@ -0,0 +1,28 @@
+using CommunityToolkit.Mvvm.Messaging;
+
+namespace Toolkit.Foundation
+{
+    public class WriteHandler<TConfiguration> : IRecipient<Write<TConfiguration>> where TConfiguration : class
+    {
+        private readonly IMessenger messenger;
+        private readonly TConfiguration configuration;
+        private readonly IConfigurationWriter<TConfiguration> writer;
+
+        public WriteHandler(TConfiguration configuration,
+            IConfigurationWriter<TConfiguration> writer,
+            IMessenger messenger)
+        {
+            this.messenger = messenger;
+            this.configuration = configuration;
+            this.writer = writer;
+        }
+
+        public void Receive(Write<TConfiguration> request)
+        {
+            request.UpdateDelegate.Invoke(configuration);
+            writer.Write(request.Section, configuration);
+
+            messenger.Send(new ConfigurationChanged<TConfiguration>(configuration));
+        }
+    }
+}