Implement Gaussian Splat schema

[cpichard]

Describe the feature you'd like
The gaussian splats schema was added in USD 26.03. https://openusd.org/release/user_guides/schemas/usdVol/ParticleField3DGaussianSplat.html
We want to at least be able to see the points in arnold when we read usd files with this schema.

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-17655