feat: add managed identity support auto-refresh app config job (#159)

* feat: add managed identity support auto-refresh app config job

* Update docs/preview/02-Features/05-AzureAppConfiguration/auto-refresh-app-configuration.md

Co-authored-by: Frederik Gheysels <[email protected]>

Co-authored-by: Frederik Gheysels <[email protected]>

Author: stijnmoreels

docs/preview/02-Features/05-AzureAppConfiguration/auto-refresh-app-configuration.md

@@ -21,14 +21,62 @@ We use the events from Azure App Configuration which will be send towards an Azu
 2. Create an Azure Service Bus Topic resource
 3. Create an event subscription on the App Configuration resource to send events to the Service Bus Topic
 
-
-
 ![Automatically refresh Azure App Configuration values](/media/Azure-App-Configuration-Job.png)
 
 Both the connection string of the Azure App Configuration and the Azure Service Bus Topic will be needed in the next section, so make sure you have those.
 
-## Usage
-Our background job requires both the configuration of Azure App Configuration while setting up the `IConfiguration`, 
+## Usage with managed identity
+When connecting to Azure App Configuration using managed identity, you only need to provide the Azure Service Bus topic name and namespace where the Azure App Configuration events are placed:
+
+```csharp
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+
+public class Program
+{
+    public static void Main(string[] args)
+    {
+        IHostBuilder hostBuilder = CreateHostBuilder(args);
+        hostBuilder.Build().Run();
+    }
+
+    private static IHostBuilder CreateHostBuilder(string[] args)
+    {
+        return Host.CreateDefaultBuilder(args)
+                    .ConfigureAppConfiguration(configBuilder => ConfigureAppConfiguration(configBuilder))
+                    .ConfigureServices(services => ConfigureServices(services));
+    }
+
+    private static void ConfigureAppConfiguration(IConfigurationBuilder configBuilder)
+    {
+        configBuilder.AddAzureAppConfiguration(appConfigOptions =>
+        {
+            appConfigOptions.Connect("<your-azure-app-configuration-endpoint>", new ManagedIdentityCredential())
+                            // Specifies which Azure App Configuration key you want to automatically updated.
+                            .Register("<your-azure-app-configuration-key>");
+        });
+    }
+
+    public void ConfigureServices(IServiceCollection services)
+    {
+        // Adds the automatic Azure App Configuration refresh background job
+        services.AddAutoRefreshAppConfigurationBackgroundJobUsingManagedIdentity(
+            // The name of the Azure Service Bus topic where Azure App Configuration events are placed.
+            topicName: "<servicebus-topic-name>",
+            // The namespace of the Azure Service Bus topic where Azure App Configuration events are placed.
+            serviceBusNamespace: "<your-namespace>.servicebus.windows.net",
+            // Prefix of the Azure Service Bus Topic subscription;
+            //    this allows the background jobs to support applications that are running multiple instances, processing the same type of events, without conflicting subscription names.
+            subscriptionPrefix: "TestSub",
+            // The client ID to authenticate for a user assigned managed identity. More information on user assigned managed identities cam be found here:
+            // https://docs.microsoft.com/en-us/azure/active-directory/managed-identities-azure-resources/overview#how-a-user-assigned-managed-identity-works-with-an-azure-vm.
+            clientId: null);
+    }
+}
+```
+
+## Usage with connection string
+When using a connection string to connect to Azure App Configuration, our background job requires both the configuration of Azure App Configuration while setting up the `IConfiguration`, 
 as well as the [Arcus secret store](https://security.arcus-azure.net/features/secret-store) which will retrieve the connection string of the Azure Service Bus Topic.
 
 ```csharp
@@ -76,4 +124,6 @@ public class Program
 }
 ```
 
-[&larr; back](/)
+## Configuration
+The background job provides configuration options to alter the behavior of the internal message pump that processes the Azure App Configuration events. 
+These messaging options are explained in detail at the [Arcus messaging feature documentation](https://messaging.arcus-azure.net/Features/message-pumps/service-bus#configuration).