[ros2] Port hand of god to ROS2

[shiveshkhaitan]

Port gazebo_ros_hand_of_god plugin to ROS2.

Example usage:

<plugin name="hand_of_god" filename="libgazebo_ros_hand_of_god.so">
  <ros>
    <namespace>/demo</namespace>
  </ros>
  <link_name>link</link_name>
  <frame_id>link</frame_id>
</plugin>

Migration guide at ROS 2 Migration: Hand of God

[chapulina]

@osrf-jenkins run tests please

[chapulina]

LGTM! I just have a couple of small comments, and it needs rebasing