[FTP/FTPS Only] Begins an asynchronous request to invoke the given site command on the server.
An IAsyncResult that references the asynchronous get.
You can create a callback method that implements the AsyncCallback delegate and pass its name to the BeginGetFeatures method. The state argument may be used to pass in data that will be passed to the callback method.
For more information please refer to the synchronous version of this method, InvokeSiteCommand.