Add documentation for Subscribe API in .NET SDK (#2606)

This commit adds an example for the subscribe API in the .NET SDK.
The .NET SDK provides two different means of using the subscirbe API
and both are included here.

https://github.com/dapr/docs/issues/2568

Signed-off-by: Hal Spang <halspang@microsoft.com>

Co-authored-by: Mark Fussell <markfussell@gmail.com>

daprdocs/content/en/developing-applications/building-blocks/configuration/howto-manage-configuration.md

@@ -39,7 +39,7 @@ Save a configuration item:
 MSET orderId1 "101||1" orderId2 "102||1"
 ```
 
-### Configure a Dapr configuration store
+## Configure a Dapr configuration store
 
 Save the following component file to the [default components folder]({{< ref "install-dapr-selfhost.md#step-5-verify-components-directory-has-been-initialized" >}}) on your machine. You can use this as the Dapr component YAML:
 
@@ -65,6 +65,7 @@ spec:
     value: <PASSWORD>
 ```
 
+## Retrieve Configuration Items
 ### Get configuration items using Dapr SDKs
 
 {{< tabs Dotnet Java Python>}}
@@ -193,7 +194,70 @@ client.GetConfigurationAlpha1({ StoreName: 'redisconfigstore', Keys = ['myconfig
 
 {{< /tabs >}}
 
-#### Watch configuration items
+### Watch configuration items using Dapr SDKs
+
+{{< tabs "Dotnet Extension" "Dotnet Client">}}
+{{% codetab %}}
+
+```csharp
+[Obsolete("Configuration API is an Alpha API. Obsolete will be removed when the API is no longer Alpha")]
+public static void Main(string[] args)
+{
+    CreateHostBuilder(args).Build().Run();
+}
+
+public static IHostBuilder CreateHostBuilder(string[] args)
+{
+    var client = new DaprClientBuilder().Build();
+    return Host.CreateDefaultBuilder(args)
+        .ConfigureAppConfiguration(config =>
+        {
+            // Get the initial value from the configuration component.
+            config.AddDaprConfigurationStore("redisconfig", new List<string>() { "withdrawVersion" }, client, TimeSpan.FromSeconds(20));
+
+            // Watch the keys in the configuration component and update it in local configurations.
+            config.AddStreamingDaprConfigurationStore("redisconfig", new List<string>() { "withdrawVersion", "source" }, client, TimeSpan.FromSeconds(20));
+        })
+        .ConfigureWebHostDefaults(webBuilder =>
+        {
+            webBuilder.UseStartup<Startup>();
+        });
+}
+```
+
+{{% /codetab %}}
+
+{{% codetab %}}
+
+```csharp
+public IDictionary<string, string> Data { get; set; } = new Dictionary<string, string>();
+public string Id { get; set; } = string.Empty;
+
+public async Task WatchConfiguration(DaprClient daprClient, string store, IReadOnlyList<string> keys, Dictionary<string, string> metadata, CancellationToken token = default)
+{
+    // Initialize the gRPC Stream that will provide configuration updates.
+    var subscribeConfigurationResponse = await daprClient.SubscribeConfiguration(store, keys, metadata, token);
+
+    // The response contains a data source which is an IAsyncEnumerable, so it can be iterated through via an awaited foreach.
+    await foreach (var items in subscribeConfigurationResponse.Source.WithCancellation(token))
+    {
+        // Each iteration from the stream can contain all the keys that were queried for, so it must be individually iterated through.
+        var data = new Dictionary<string, string>(Data);
+        foreach (var item in items)
+        {
+            // The Id in the response is used to unsubscribe.
+            Id = subscribeConfigurationResponse.Id;
+            data[item.Key] = item.Value;
+        }
+        Data = data;
+    }
+}
+```
+
+{{% /codetab %}}
+{{< /tabs >}}
+
+### Watch configuration items using gRPC API
 
 Create a Dapr gRPC client from the [Dapr proto](https://github.com/dapr/dapr/blob/master/dapr/proto/runtime/v1/dapr.proto) using your [preferred language](https://grpc.io/docs/languages/). Use the `SubscribeConfigurationAlpha1` proto method on your client stub to start subscribing to events. The method accepts the following request object: