Adds option for terrain border to above or below the ground

[pascal-roth]

Description

Adds option to add terrain border as "wall" above the ground or as terrain extension below the ground in TerrainGeneratorCfg.

Type of change

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

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