[Feature] Surface normals of Gaussian Splats

[Crezle]

Is your feature request related to a problem? Please describe.
The feature is requested as splat export format includes "nx, ny, nz" fields but are all zero. There are currently (f604fd9) no options for exporting splats with normals.

Describe the solution you'd like
I would guess this should be possible due to gsplat having normal outputs, so eventually an option to include normals the same way that can be done for NeRF pointclouds.

Describe alternatives you've considered
I've considered just using gsplat directly for retrieving these.