Before AJAX toolkit works as-is, we need to port JavaScript Serializer as we don't want to touch the AJAX toolkit and change the serializer. So below PR ports the JS serializer #403 .

Here are the foundations,

• Ported public API's to keep the backward compatibility as is. But under the hood changed to use STJ(System.Text.JSON)
• Added Serializer Converter Factory to ensure RegisterConverter (backward API) works else default to STJ.
• Added Custom Deserializer to ensure backward compatibility works, else default to STJ.
• ObjectConverter class ported to keep backward compatible.
• Added Test project with unit tests. (Let's keep adding more unit tests as we write more code)