Add examples on how to implement a timeout mechanism in methods GetResponseAsync/GetRequestStreamAsync for System.Net.FileWebRequest

As a reference, [`HttpWebRequest.BeginGetResponse`](https://docs.microsoft.com/en-us/dotnet/api/system.net.httpwebrequest.begingetresponse?view=netcore-3.0) already contains an example on how to implement such logic, the point is to look for something that can work with methods GetResponseAsync/GetRequestStreamAsync.