v4041: Add GetKey method that works with the license server

In this update, we have introduced a new version of GetKey method that is compatible with the license server and tailored to be used in cross platform environments. If used to connect to the license server, it provides a way to list all activated machines when a local floating license server is used (https://github.com/Cryptolens/license-server/blob/master/README.md#floating-licenses-offline).

An example of how this method can be used is shown below:

var result = Key.GetKey(token: "", productId: 3349, key: "", LicenseServerUrl: "http://192.168.0.2:8080");

// obtaining the license key (and verifying the signature automatically).
var license = LicenseKey.FromResponse("RSAPubKey", result);