Align message definitions to ROS equivalents

[azeey]

It's very common to use Gazebo with ROS and currently, this is done using the ros_gz_bridge which is a ROS node that converts gz-transport messages to ROS messages and vice-versa. To optimize this use case, it would be beneficial to align ROS and Gazebo message definitions so that the conversions are trivial, and more conducive for automating the creation of message converters.