Allow updating robot gravity

[AdamPettinger]

I have a UR20 mounted on another robot that can change its orientation enough to cause problems for the arm's mounting installation.

There has been some discussion on how to update the mounting orientation/gravity live (here and here), but these did not end up happening.

These changes expose the set_gravity URScript functionality through the ROS driver, allowing a user to update the gravity vector of the robot during live operation via a service. This is obviously risky if you calculate the gravity vector incorrectly.

There are corresponding changes in

• https://github.com/UniversalRobots/Universal_Robots_Client_Library/pull/415
• https://github.com/ros-industrial/ur_msgs/pull/39

I just rebased these commits on main today, although we have been running these changes forked from the jazzy branch for a few months now.