Implement getNodeNames from rcl_get_node_names

[mattrichard]

Exposes the rcl rcl_get_node_names function as part of the rclnodejs Node class. Unlike the rclpy lib which returns an array of tuples, getNodeNames will return an array of objects following the pattern used in the other getXNamesAndTypes functions.

Here's some example output.

[
  { name: '_ros2cli_daemon_0', namespace: '/' },
  { name: 'launch_ros_4923', namespace: '/' },
  { name: 'robot_state_publisher', namespace: '/' },
  { name: 'gazebo', namespace: '/' },
  { name: 'turtlebot3_imu', namespace: '/' },
  { name: 'turtlebot3_laserscan', namespace: '/' },
  { name: 'camera_driver', namespace: '/' },
  { name: 'turtlebot3_diff_drive', namespace: '/' },
  { name: 'turtlebot3_joint_state', namespace: '/' }
]

Fix #555

[minggangw]

Thanks for submitting this PR, I checked the error output on circleCI

Error: dlopen(/Users/distiller/RobotWebTools/rclnodejs/build/Release/rclnodejs.node, 1): Library not loaded: /usr/local/opt/poco/lib/libPocoFoundation.63.dylib

The reason is that the ROS2 binary is built with a different version of poco, which happened before. So, it's not your code causing this. Thanks for writing a unit test too and the PR looks good to me, I am going to merge it soon after some further checking.

P.S. there is an issue (#498) used to track the absent features, I think I missed this out.

[minggangw]

Thanks for submitting it, LGTM.