Add AdditionalServiceRegistration action for custom ASP.NET Core DI setup (#611)

src/WireMock.Net/Owin/IWireMockMiddlewareOptions.cs

@@ -8,6 +8,7 @@ using WireMock.Util;
 using Owin;
 #else
 using IAppBuilder = Microsoft.AspNetCore.Builder.IApplicationBuilder;
+using Microsoft.Extensions.DependencyInjection;
 #endif
 
 namespace WireMock.Owin
@@ -36,6 +37,10 @@ namespace WireMock.Owin
 
         Action<IAppBuilder> PostWireMockMiddlewareInit { get; set; }
 
+#if USE_ASPNETCORE
+        Action<IServiceCollection> AdditionalServiceRegistration { get; set; }
+#endif
+
         IFileSystemHandler FileSystemHandler { get; set; }
 
         bool? AllowBodyForAllHttpMethods { get; set; }