Add "is in remote control" call as a dashboard service

[urfeex]

Feature summary

As discussed in #1191 it would be nice to have a service for querying the remote_control mode state. This is implemented in the dashboard server and in the ur_client_library, but not in the ROS wrapper.

Related issues

ur_client_library API call: https://github.com/UniversalRobots/Universal_Robots_Client_Library/blob/4daad9b890943cfc22bee74df7ac8a069ca3ed6e/include/ur_client_library/ur/dashboard_client.h#L266

Tasks

To complete this issue involves

• Implement the feature
• Make documentation
• Make Unit test
• Test on real hardware