Adds environment for lifting a deformable cube

[masoudmoghani]

Description

Add soft cube RL env.

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