Add ability to describe multiple arms using xs_launch

[lukeschmitt-tr]

This PR adds the ability to create multiple robot descriptions through the xs_launch module's DeclareInterbotixXSArmRobotDescriptionLaunchArgument class and declare_interbotix_xsarm_robot_description_launch_arguments method.

To do so, one can pass in custom names to declare_interbotix_xsarm_robot_description_launch_arguments for the following launch configuration/arguments for each robot to be launched:

• robot_model_launch_config_name
• robot_name_launch_config_name
• robot_description_launch_config_name

These names are passed down to the DeclareInterbotixXSArmRobotDescriptionLaunchArgument class which creates a custom-named robot description LaunchConfiguration that can be accessed in launch files.