Add support for EXT_structural_metadata extension

[gkjohnson]

Specification

https://github.com/CesiumGS/glTF/tree/3d-tiles-next/extensions/2.0/Vendor/EXT_structural_metadata

Test Files

https://github.com/CesiumGS/3d-tiles-samples/tree/main/glTF/EXT_structural_metadata

TODO

• Implement #526 first
• Add support for extension
• Add demo page

[gkjohnson]

Property Tables

• Requires Id to access, either from mesh feature ids or included node data

Property Textures

• Requires association with a primitive

Property Attributes

• Requires association with a primitive
• Must be points

[gkjohnson]

Structure:

Add property table accessor for each node.

Add helpers for accessing limited accessors for nodes that require it.

Consider removing array index-by-index accessors for now.

[gkjohnson]

Demo

• Provide hover states based on feature id / table name
• Provide heatmap visualizations based on textures, attributes
• Adjust highlighting of the feature ids based on property table