Decouples physics backends from SimulationContext via PhysicsManager

[ooctipus]

Description

Refactors SimulationContext to use a PhysicsManager abstraction layer, enabling cleaner separation between simulation orchestration and physics backend implementation. Introduces backward-compatible deprecation for configuration fields that moved to PhysxCfg.

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

Checklist

• I have read and understood the contribution guidelines
• 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

[greptile-apps]

Too many files changed for review. (183 files found, 100 file limit)