feat: spring bones

[daniele-dcl]

Summary

Adds support for the DCL_spring_bone_joint GLTF extension during asset bundle conversion, enabling spring bone physics on converted wearables and emotes.

What this does

• Integrates spring bone processing into the GLTF import pipeline: During the PreProcessGameObjects phase of the custom GLTF importer, spring bone joint data is now extracted from the GLTF source and applied as SpringBoneJointComponent components on the corresponding GameObjects.
• Bumps the unity-gltf submodule to include:
  • Parsing of the DCL_spring_bone_joint GLTF extension (stiffness, drag, gravity direction/power, hit radius, root flag)
  • Serializable spring bone component properties for runtime use
  • A reusable SpringBoneUtils utility that resolves spring bone data from the GLTF node tree and maps it onto the Unity scene hierarchy by node name

How it works

1. The GLTF file is parsed by unity-gltf, which now recognizes the DCL_spring_bone_joint extension on nodes.
2. After the scene hierarchy is instantiated, CustomGltfImporter.PreProcessGameObjects calls SpringBoneUtils.ApplySpringBoneJoints, passing the parsed GLTF data and the root GameObject.
3. The utility iterates over all GLTF nodes, finds those with spring bone extension data, matches them to scene transforms by name, and attaches a SpringBoneJointComponent with the configured physics parameters.