Requests for additional 3D Tiles samples

This issue is intended for collecting high-level ideas about 3D Tiles samples that may be added here. Details about specific examples can be discussed in dedicated issues or the corresponding PRs.

^{(This issue is a replacement of https://github.com/CesiumGS/3d-tiles/issues/562 , and the following list is based on this issue)}

- Simple "unit test" models for the major new features
- Simple "unit test" models for the major types of models, e.g., photogrammetry, CAD, point clouds
- "Integration test" models that combine prominent use cases such as per-texel metadata in photogrammetry, and MESHOPT-compressed point clouds
- "Scalability test" model so developers can test their out of core algorithms
- A double-headed quadtree (see https://github.com/CesiumGS/3d-tiles/issues/512#issuecomment-956636721 )
- Further examples of implicit tiling, maybe pointing out different combinations of available levels and subtree levels...?
- A small sample with `3DTILES_bounding_volume_S2`
- Samples for the 'semantics' of metadata
- More examples for the glTF Metadata extensions. For example, there could be an `EXT_structural_metadata` version of the current [`TilesetWithFullMetadata`](https://github.com/CesiumGS/3d-tiles-samples/tree/29ad5b6c9f3932d7483023d2b48e4ba4662164cd/1.1/TilesetWithFullMetadata)

Start by reviewing the requested sample categories and the existing 1.1/TilesetWithFullMetadata sample. This issue is only a high-level collection of ideas, so choose a specific example and move its details into a dedicated issue or PR. Done should be defined there for the selected sample.

Written by the indexing model from the issue text.