AnimationClip: Add userData and honor in glTF loader and exporter.

[repalash]

Description

glTF schema supports extras property in animations. But when loading a file with extra properties using GLTFLoader and saving it with GLTFExporter, these properties are lost.

This PR adds support by adding userData to AnimationClip and loading the animation extras in that, similar to objects, materials, which is then used in GLTFExporter. Also this is useful to store any custom data like in the threejs editor.

This contribution is funded by Threepipe

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.63 79.03	338.63 79.03	+0 B +0 B
WebGPU	567.9 157.11	567.9 157.11	+0 B +0 B
WebGPU Nodes	566.51 156.86	566.51 156.86	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	470.38 113.78	470.38 113.78	+0 B +0 B
WebGPU	641.64 173.58	641.64 173.58	+0 B +0 B
WebGPU Nodes	596.28 162.88	596.28 162.88	+0 B +0 B