Official beta release of companion lib for Flurl.Http 4.0+ that allows Newtonsoft.Json to continue to be used despite its removal, easing backward compatibility with 3.x.

• Brings back NewtonsoftJsonSerializer from 3.x for use in 4.0 and beyond
• Brings back non-generic, dynamic-returning extension methods from 3.x:
  • GetJsonAsync() and GetJsonListAsync()
  • ReceiveJson() and ReceiveJsonList()
  • FlurlHttpException.GetResponseJsonAsync
• Easy registration via IFlurlClientBuilder.UseNewtonsoft() and IFlurlClientCache.UseNewtonsoft()