Feature request

Feature description

In ROS 1 rosmsg show would show you the recursively expanded message fields, whereas in ROS 2 at the moment ros2 interface show only shows you the fields of the immediate message you specify.

For example, in ROS 1 (from https://wiki.ros.org/ROS/Tutorials/UnderstandingTopics#Using_rostopic_type):

$ rosmsg show geometry_msgs/Twist
geometry_msgs/Vector3 linear
  float64 x
  float64 y
  float64 z
geometry_msgs/Vector3 angular
  float64 x
  float64 y
  float64 z

And the equivalent in ROS 2:

% ros2 interface show geometry_msgs/msg/Twist
# This expresses velocity in free space broken into its linear and angular parts.

Vector3  linear
Vector3  angular

It does, however, show the comments in the message file, which may be nice to have some times.

Also, as a side note, this command takes almost 2 seconds to run on my macbook, but I'm not sure why...