Add graceful shutdown

[achim-k]

Public API Changes
None

Description
This patch adds graceful shutdown for the rosapi/rosapi_node and rosbridge_server/rosbridge_websocket.py nodes when a SIGINT is recieved (e.g. via ctrl+c). rclpy by default adds an sigint handler which takes care of the shutdown already.

Without this patch, the following exception logs are printed:

[rosbridge_websocket-1] Traceback (most recent call last):
[rosbridge_websocket-1]   File "/opt/ros2_ws/install/rosbridge_server/lib/rosbridge_server/rosbridge_websocket", line 343, in <module>
[rosbridge_websocket-1]     main()
[rosbridge_websocket-1]   File "/opt/ros2_ws/install/rosbridge_server/lib/rosbridge_server/rosbridge_websocket", line 338, in main
[rosbridge_websocket-1]     rclpy.shutdown()
[rosbridge_websocket-1]   File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/__init__.py", line 126, in shutdown
[rosbridge_websocket-1]     _shutdown(context=context)
[rosbridge_websocket-1]   File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/utilities.py", line 58, in shutdown
[rosbridge_websocket-1]     return context.shutdown()
[rosbridge_websocket-1]   File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/context.py", line 102, in shutdown
[rosbridge_websocket-1]     self.__context.shutdown()
[rosbridge_websocket-1] rclpy._rclpy_pybind11.RCLError: failed to shutdown: rcl_shutdown already called on the given context, at ./src/rcl/init.c:241
[rosapi_node-2] [INFO] [1664897157.258284532] [rosapi]: Exiting due to SIGINT
[rosapi_node-2] Failed to publish log message to rosout: publisher's context is invalid, at ./src/rcl/publisher.c:389
[rosapi_node-2] Traceback (most recent call last):
[rosapi_node-2]   File "/opt/ros2_ws/install/rosapi/lib/rosapi/rosapi_node", line 330, in <module>
[rosapi_node-2]     main()
[rosapi_node-2]   File "/opt/ros2_ws/install/rosapi/lib/rosapi/rosapi_node", line 326, in main
[rosapi_node-2]     rclpy.shutdown()
[rosapi_node-2]   File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/__init__.py", line 126, in shutdown
[rosapi_node-2]     _shutdown(context=context)
[rosapi_node-2]   File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/utilities.py", line 58, in shutdown
[rosapi_node-2]     return context.shutdown()
[rosapi_node-2]   File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/context.py", line 102, in shutdown
[rosapi_node-2]     self.__context.shutdown()
[rosapi_node-2] rclpy._rclpy_pybind11.RCLError: failed to shutdown: rcl_shutdown already called on the given context, at ./src/rcl/init.c:241