Ability to set a robot_description that is not within the drake_models package

[DaniGarciaLopez]

Addresses #68

Example of usage

Add the following in drake_ktopt_planning.yaml:

base_frame: ""
external_robot_description: ["/home/user/xarm_ws/src/xarm_ros2/xarm_description"]

[kamiradi]

Thanks for the PR,

Could you add a section in the readme with your example usage. Your PR message should be just fine. Also, follow the pre-commit steps so that the formatting is in order.

Thanks!

[DaniGarciaLopez]

Thanks for reviewing! Not sure where was the best place to add it. What about something like that?

[kamiradi]

Yup,

This looks good for now.