Added AsyncRequestHandle class, to support request cancel in future.

In case of implementing queue of requests I need handle to async request. It will allow to cancel them later on.

Referenced from: https://github.com/restsharp/RestSharp/blob/master/RestSharp/RestRequestAsyncHandle.cs
https://github.com/restsharp/RestSharp/blob/master/RestSharp/RestClient.Async.cs