[Proposal] Default metadata in BaseMujocoEnv

[spiglerg]

Description

The current code for BaseMujocoEnv requires the env metadata dictionary to have fixed, pre-specified values. While this may be useful for future API changes, it doesn't seem very useful at the moment.

Fixes #1059

Type of change

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

Checklist:

• [x] I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] New and existing unit tests pass locally with my changes