Add the new reward for gait, which is based on the feet body name

[farbod-farshidian]

Description

It improves over-the-spot gait reward by using body names instead of indices.

Fixes # (issue)

Type of change

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

Screenshots

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

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 run all the tests with ./isaaclab.sh --test and they pass
• 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