Add support for KHR_draco_mesh_compression in glTF/glb export

## Feature request: Draco mesh compression on glTF/glb export

**Background**
From what I understand, the USD_FILEFORMATS_ENABLE_DRACO CMake option currently enables reading Draco-compressed meshes on import. However, there appears to be no support for exporting glTF files with KHR_draco_mesh_compression.

**Use case**
- Large USD scenes generate very large glTFs; mesh data dominates file size.
- Draco compression could reduce mesh payloads by ~60–80%, speeding up delivery in web and real-time viewers.

**Proposal**
Introduce `enableDracoCompression` (or similar) in `ExportGltfOptions`.

**Alternatives considered**
- Post-processing with external tools, but integrating at export avoids extra steps in our USD-centric pipelines.

*Thanks for considering this enhancement!*