Dabonnie/add test header message

[dabonnie]

Add test message needed by ros2/rclcpp#1080

[dabonnie]

FYI @wjwwood @jacobperron

[jacobperron]

We can't accept this PR as it introduces a circular dependency at the repository level: test_msgs (rcl_interfaces) -> std_msgs (common_interfaces) -> builtin_interfaces (rcl_interfaces).

[dabonnie]

Closing this as we'll open another with your proposal (copy the message structure rather than use a dependency).

[jacobperron]

Closing this as we'll open another with your proposal (copy the message structure rather than use a dependency).

Please consider generating the message directly in rclcpp. I think it will be much neater (and easily refactored) versus adding a new public message to test_msgs.