Moves creating Trimesh from USD prim to an util function

[Mayankm96]

Description

Previously, the TerrainImporter and RayCaster were storing a copy of the meshes added to the simulation. While we now remove the meshes from the terrain importer, we still want to keep the option to read meshes from the USD or at least provide a utility for it. This might be helpful in places where users want to generate commands on rough terrains.

This MR moves the reading USD mesh prims from the RayCaster into its own function.It simplifies the ray caster code and also does the job as a util function.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there