Expose GLTFInstance.userData in the API

### Description

It would be useful to have access to custom data embedded in the GLTF file. ThreeJS appears to save this in a [`userData`](https://threejs.org/docs/#api/en/core/Object3D.userData) property, which is already being used in this project for the variant logic. My client uses \ to show models that change appearance based on some user input. The `userData` could be utlized to store information on how the user input needs to be processed for a particular model or material. Another use-case might be to store the annotation data right in the GLTF file.

I would be happy to implement this if this is something that is deemed in-scope for this project.

### Version

- model-viewer: v3.5.0