Add foundation

Toolkit.Foundation/AppService.cs

@@ -0,0 +1,20 @@
+using Microsoft.Extensions.Hosting;
+
+namespace Toolkit.Foundation;
+
+public class AppService(IEnumerable<IInitializer> initializers,
+    IPublisher publisher) :
+    IHostedService
+{
+    public async Task StartAsync(CancellationToken cancellationToken)
+    {
+        foreach (IInitializer initializer in initializers)
+        {
+            await initializer.Initialize();
+        }
+
+        await publisher.Publish<Started>(cancellationToken);
+    }
+
+    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+}