C# HttpClient

Hi,

It looks like only RestSharp is here for C#. [HttpClient](https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclient?view=netcore-2.2) is in the .NET standard library and is the standard way to make HTTP requests (without a third party library like RestSharp).

If there is no ongoing work to add HttpClient, I can PR to add a snippet for HttpClient.

Thanks.