Transition to `Unity.Plastic.Newtonsoft.Json`

[paulhazen]

This PR introduces use of the already included Unity.Plastic.Newtonsoft.Json package. This does not actually introduce a dependency for the project but makes use of the Unity package version of Newtonsoft that is included already in the project. (Prior attempts to introduce Newtonsoft failed because we were trying to introduce the library independent of the Unity package system).

This PR also moves the JsonUtility implementation away from using UnityEngine.JsonUtility under the covers, and instead makes use of the aforementioned Unity.Plastic.Newtonsoft.Json library for implementation.