Make use of sync HttpClient API in HttpWebRequest

[ManickaP]

https://github.com/dotnet/runtime/blob/master/src/libraries/System.Net.Requests/src/System/Net/HttpWebRequest.cs#L1022

https://github.com/dotnet/runtime/blob/master/src/libraries/System.Net.Requests/src/System/Net/HttpWebResponse.cs#L352

cc: @stephentoub

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