Allow selecting articulation root prim explicitly

[lgulich]

Description

Normally the Articulation class would search for an articulation root below ArticulationCfg.prim_path. However this fails when multiple articulation roots are present. This PR allows to explicitly specify an articulation root prim path with ArticulationCfg.articulation_root_prim_path. If not set it will fall back to the old search approach.

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

[jtigue-bdai]

In your use case, are you importing a USD with multiple robots? If so I don't think the Articulation root_physx_view will initialize for a multirobot setup.