Add support for spawning meshes from obj and stl files

[han-xudong]

Description

Adding support for spawning meshes from OBJ or STL files since current version only supports basic geometries. The required dependency is trimesh which is already included.

Fixes #2378

Type of change

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

Screenshots

Please attach before and after screenshots of the change if applicable.

Checklist

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