Expose underlying JSON

It would be very convenient if the underlying JSON strings were exposed by the various model classes.
Then I could do things like `Debug.WriteLine(myBoxEnterpriseEvent.RawJson)`.

Start by locating the various model classes and the JSON deserialization entry point in the SDK. Expose each model's underlying JSON through a RawJson property, then verify that a Box model such as BoxEnterpriseEvent returns the original JSON string.

Written by the indexing model from the issue text.