Update 3DTILES_implicit_tiling and 3DTILES_subtree

[lilleyse]

Updates 3DTILES_implicit_tiling and 3DTILES_subtree.

There are two main differences between this and 3D Tiles 1.1:

1. Tile semantics such as TILE_BOUNDING_VOLUME that were previously stored in a property table are now stored in an attributes object. This helps distinguish structural attributes from application-specific properties. More motivation behind that here: Semantics vs. extensions 3d-tiles#782 (comment).
2. Now that this is built on glTF, we can use EXT_structural_metadata to store application-specific properties.

Example of what it looks like now:

"3DTILES_subtree": {
  "tileAvailability": {
    "constant": 1
  },
  "contentAvailability": [{
    "bitstream": 0,
    "availableCount": 60
  }],
  "childSubtreeAvailability": {
    "bitstream": 1
  },
  "tileAttributes": {
    "TILE_BOUNDING_BOX": 0, // points to an accessor
    "TILE_GEOMETRIC_ERROR": 1
  },
  "contentAttributes": {
    "CONTENT_BOUNDING_BOX": 2
  },
  "tilePropertyTable": 0, // points to a property table in EXT_structural_metadata
  "contentPropertyTable": 1
}

There is still a TODO list at the bottom of each extension, but I think this could be merged as a first draft.