improve support for loading glTF "extras"

[stephengold]

Currently, the loaded data structure allows metadata to be attached to a scene or a node. When loading a glTF asset that contains "extras", Assimp attempts to convert node-level extras to node-level metadata.

The glTF 2.0 specification also allows for extras to be attached to accessors, animations, buffers, cameras, images, materials, meshes, samplers, skins, and textures. However, the corresponding Assimp data structures don't provide anywhere to load this information.

Is your feature request related to a problem? Please describe.
Information is lost when loading glTF models.

Describe the solution you'd like
This issue requests that metadata members of type aiMetadata * be added to struct aiMesh, struct aiCamera, struct aiMaterial, and so on, and that the glTF 2.0 loader be enhanced to populate the appropriate members whenever "extras" properties are present in the glTF.