Allow registering the WhatsApp handler separately

Add another overload that doesn't register any handler type.

Author: kzu

src/WhatsApp/AzureFunctionsExtensions.cs

@@ -57,6 +57,15 @@ static void ConfigureServices(IServiceCollection services)
             .ValidateDataAnnotations();
     }
 
+    /// <summary>
+    /// Configure WhatsApp functions and use an already registered service that implements <see cref="IWhatsAppHandler"/>.
+    /// </summary>
+    public static IFunctionsWorkerApplicationBuilder UseWhatsApp(this IFunctionsWorkerApplicationBuilder builder)
+    {
+        ConfigureServices(builder.Services);
+        return builder;
+    }
+
     /// <summary>
     /// Configure the WhatsApp handler for Azure Functions.
     /// </summary>