Updated to dotnet standard

[jamesr981]

This library was originally written in dotnet framework.
As a result, integrators are unable to use this library unless they are also using dotnet framework.

To allow for both dotnet framework and Dotnet, I have migrated the Dotnet client library to Dotnet Standard.

This involved a slight refactor to Json Deserialization.
Previously, a class JavascriptSerializer was being used. But this is exclusive to dotnet framwork.
Instead, I have installed System.Text.Json and created a little converter that handles the differences in deserialization between JavascriptSerializer and System.Text.Json.

As a result, there should be no breaking changes in this upgrade, but a major version bump should be done.

Not all of the unit tests passed for me as the credentials I have do not grant access to all the endpoints.