[Feature] Scene API - first pass

[kanerogers]

Background

Part of #315

A very simple way to get this going is:

{
  "scene_name": "Simple Scene Example",
  "entities": [
    {
      "name": "Damaged Helmet",
      "id": 0,
      "model": "damaged_helmet.glb",
      "transform": {
        "translation": [0.0, 1.4, 0.0],
        "scale": [1.0, 1.0, 1.0],
        "rotation": [0.0, 90.0, 0.0], // yes, euler degrees
      }
    }
  ]
}