Make kestrel limits configurable (#520)

* Allow kestrel options to be overriden with values of config files and environment variables (see https://docs.microsoft.com/en-us/aspnet/core/fundamentals/servers/kestrel?view=aspnetcore-3.1#kestrel-options)

* Make appsettings.json optional

* Move changes to AspNetCoreSelfHost.Framework files.

* Implement ConfigureKestrelServerOptions for .NET Standard 1.3.

src/WireMock.Net/Owin/AspNetCoreSelfHost.NETStandard.cs

@@ -3,6 +3,8 @@ using System;
 using System.Collections.Generic;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Server.Kestrel.Core;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
 
 namespace WireMock.Owin
 {
@@ -34,5 +36,24 @@ namespace WireMock.Owin
             }
         }
     }
+
+    internal static class IWebHostBuilderExtensions
+    {
+        internal static IWebHostBuilder ConfigureAppConfigurationUsingEnvironmentVariables(this IWebHostBuilder builder)
+        {
+            return builder.ConfigureAppConfiguration(config =>
+            {
+                config.AddEnvironmentVariables();
+            });
+        }
+
+        internal static IWebHostBuilder ConfigureKestrelServerOptions(this IWebHostBuilder builder)
+        {
+            return builder.ConfigureServices((context, services) =>
+            {
+                services.Configure<KestrelServerOptions>(context.Configuration.GetSection("Kestrel"));
+            });
+        }
+    }
 }
 #endif