The default plane configuration supports setting the plane color, but
this option is not currently exposed via the terrain importer. This PR
adds a ``color'' field to the terrain importer that is only used for
configuring a plane.

- New feature (non-breaking change which adds functionality)

<img width="912" alt="image"
src="https://github.com/user-attachments/assets/4e509270-a953-49e8-88e7-b67bc1a92d2c"
/>

<img width="909" alt="image"
src="https://github.com/user-attachments/assets/0aa77530-befe-4c30-991d-e668561b9793"
/>

Configuration:

```python
terrain = TerrainImporterCfg(
            prim_path="/World/ground",
            terrain_type="plane",
            terrain_generator=None,
            max_init_terrain_level=None,
            collision_group=-1,
            physics_material=sim_utils.RigidBodyMaterialCfg(
                friction_combine_mode="multiply",
                restitution_combine_mode="multiply",
                static_friction=1.0,
                dynamic_friction=1.0,
            ),
            color=(1.0, 0.0, 0.0), # Red
            debug_vis=False,
        )
```

- [ ] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [X] 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
- [X] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

---------
Signed-off-by: Kyle Morgenstein <34984693+KyleM73@users.noreply.github.com>
Signed-off-by: Mayank Mittal <12863862+Mayankm96@users.noreply.github.com>
Co-authored-by: Mayank Mittal <12863862+Mayankm96@users.noreply.github.com>