Adds 3DTILES_feature_metadata extension

[lilleyse]

Direct Link	Draft Version	Changes
3DTILES_feature_metadata	0.0.0	Initial draft

This extension lists the feature types present in a tileset and provides a mechanism for storing feature metadata alongside tile content (glTF or non-glTF content). Feature metadata mirrors the EXT_feature_metadata glTF extension.

The top-level 3DTILES_feature_metadata extension lists feature types that are referenced by tiles within the tileset. A client may use this information to populate a UI, filter metadata requests, or generate styles on-the-fly.

To do:

• Clarify relationship between glTF feature metadata, sidecar feature metadata, and 3D Tiles feature metadata
• Add implicit tiling example
• Add ability to point to an external file for feature type definitions
  • Can this external file include feature types not present in this tileset JSON?

[lilleyse]

Closing for now since class definitions are now stored in 3DTILES_metadata and there's still many open questions about how metadata is stored for non glTF content.