Fix WebApps OpenAI usage (#269)

The Kernel was never registered in DI, so it wasn't possible to use the AI services.

src/WebApp/Extensions/Extensions.cs

@@ -103,6 +103,7 @@ private static void AddAIServices(this IHostApplicationBuilder builder)
 
         if (!string.IsNullOrWhiteSpace(builder.Configuration.GetConnectionString("openai")) && !string.IsNullOrWhiteSpace(deploymentName))
         {
+            builder.Services.AddKernel();
             builder.AddAzureOpenAI("openai");
             builder.Services.AddAzureOpenAIChatCompletion(deploymentName);
         }