add spacenav example

[borongyuan]

Description

Add a simple spacenav example, only launch file. Need to wait for ros-drivers/joystick_drivers#251 to be merged. There I added an option to make spacenav_node publish TwistStamped directly.

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[vatanaksoytezer]

@borongyuan thank you very much for your contribution! I think the place for this example is https://github.com/ros-planning/moveit2_tutorials/tree/main/doc/examples/realtime_servo. Can you close this PR and open one for moveit2_tutorials?

[borongyuan]

Ah, I see. I will move this to another repo.