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.cs

@@ -58,6 +58,7 @@ namespace WireMock.Owin
             }
 
             _host = builder
+                .ConfigureAppConfigurationUsingEnvironmentVariables()
                 .ConfigureServices(services =>
                 {
                     services.AddSingleton(_options);
@@ -81,6 +82,7 @@ namespace WireMock.Owin
 
                     SetHttpsAndUrls(options, _urlOptions.GetDetails());
                 })
+                .ConfigureKestrelServerOptions()
 
 #if NETSTANDARD1_3
                 .UseUrls(_urlOptions.GetDetails().Select(u => u.Url).ToArray())