Fix lazy physics preset imports

[AntoineRichard]

Summary

• Added generic lazy preset support for config alternatives backed by import paths.
• Moved direct task OV-PhysX physics presets to lazy imports so default config loading no longer imports isaaclab_ovphysx.
• Added regression coverage for lazy preset discovery, selection, and missing-package errors.

Test Plan

• ./isaaclab.sh -f
• ./isaaclab.sh -p -m pre_commit run --files source/isaaclab_tasks/isaaclab_tasks/utils/hydra.py source/isaaclab_tasks/test/test_hydra.py source/isaaclab_tasks/isaaclab_tasks/direct/cartpole/cartpole_env_cfg.py source/isaaclab_tasks/isaaclab_tasks/direct/ant/ant_env_cfg.py source/isaaclab_tasks/isaaclab_tasks/direct/humanoid/humanoid_env_cfg.py source/isaaclab_tasks/isaaclab_tasks/direct/locomotion/locomotion_env.py source/isaaclab_tasks/docs/CHANGELOG.rst source/isaaclab_tasks/config/extension.toml source/isaaclab_tasks/isaaclab_tasks/utils/init.pyi
• PYTHONPYCACHEPREFIX=/tmp/isaaclab_pycache ./isaaclab.sh -p -m py_compile source/isaaclab_tasks/isaaclab_tasks/utils/hydra.py source/isaaclab_tasks/isaaclab_tasks/direct/cartpole/cartpole_env_cfg.py source/isaaclab_tasks/isaaclab_tasks/direct/ant/ant_env_cfg.py source/isaaclab_tasks/isaaclab_tasks/direct/humanoid/humanoid_env_cfg.py source/isaaclab_tasks/isaaclab_tasks/direct/locomotion/locomotion_env.py source/isaaclab_tasks/test/test_hydra.py

Not Run

• PYTHONPYCACHEPREFIX=/tmp/isaaclab_pycache ./isaaclab.sh -p -m pytest source/isaaclab_tasks/test/test_hydra.py -q: blocked because the /usr/bin/python3.12 environment is missing torch.
• ./isaaclab.sh -d: blocked by PEP 668 externally-managed system Python while installing docs requirements.