feat(robot): add support for handling scalar joint positions
- Updated `RobotData` and `RbsArm` to allow joint positions to be passed
as either a list or a scalar value.
- Refactored logic to dynamically handle scalar values and repeat them
across all joints.
- Added new `initial_arm_joint_positions` and
`initial_gripper_joint_positions` properties in `RbsArm`.
- Improved joint position initialization in the `Scene` class to
retrieve robot description asynchronously if not provided.
- Enhanced the launch script to configure environment variables based on
available CPU cores.
- Minor code cleanups and bug fixes, including terrain naming and
corrected joint defaultsd.
git commit --amend
