Feature Request: Support for Vec4 Metadata Types

Feature Request: Support for Vec4 Metadata Types: a task in LegoFlow-SWE (Harbor dataset). Problem OpenVDB currently provides dedicated metadata types for 3‑component vectors ( Vec3IMetadata , Vec3SMetadata , Vec3DMetadata ), but equivalent support for 4‑component vectors ( Vec4i , Vec4s , Vec4d )…

The task

**Problem** OpenVDB currently provides dedicated metadata types for 3‑component vectors (`Vec3IMetadata`, `Vec3SMetadata`, `Vec3DMetadata`), but equivalent support for 4‑component vectors (`Vec4i`, `Vec4s`, `Vec4d`) is missing. Users need to attach 4‑component vector data (e.g., RGBA colors, homogeneous coordinates)…

Part of Lego-X/LegoFlow-SWE.