Add DI extensibility note to durable HTTP article

Author: cgillum

articles/azure-functions/durable/durable-functions-http-features.md

@@ -206,6 +206,38 @@ If any of these limitations might affect your use case, consider instead using a
 >
 > This design choice is intentional. The primary reason is that custom types help ensure users don't make incorrect assumptions about the supported behaviors of the internal HTTP client. Types specific to Durable Functions also make it possible to simplify API design. They also can more easily make available special features like [managed identity integration](#managed-identities) and the [polling consumer pattern](#http-202-handling). 
 
+### Extensibility (.NET only)
+
+Customizing the behavior of the orchestration's internal HTTP client is possible using [Azure Functions .NET dependency injection](https://docs.microsoft.com/azure/azure-functions/functions-dotnet-dependency-injection). This ability can be useful for making small behavioral changes. It can also be useful for unit testing the HTTP client by injecting mock objects.
+
+The following example demonstrates using dependency injection to disable SSL certificate validation for orchestrator functions that call external HTTP endpoints.
+
+```csharp
+public class Startup : FunctionsStartup
+{
+    public override void Configure(IFunctionsHostBuilder builder)
+    {
+        // Register own factory
+        builder.Services.AddSingleton<
+            IDurableHttpMessageHandlerFactory,
+            MyDurableHttpMessageHandlerFactory>();
+    }
+}
+
+public class MyDurableHttpMessageHandlerFactory : IDurableHttpMessageHandlerFactory
+{
+    public HttpMessageHandler CreateHttpMessageHandler()
+    {
+        // Disable SSL certificate validation (not recommended in production!)
+        return new HttpClientHandler
+        {
+            ServerCertificateCustomValidationCallback =
+                HttpClientHandler.DangerousAcceptAnyServerCertificateValidator,
+        };
+    }
+}
+```
+
 ## Next steps
 
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