Move example messages out of std_msgs

[tfoote]

This is a follow up to the Aggregate Foxy Message API Review

Followup to: https://github.com/ros2/common_interfaces/pull/86/files#r392505128

We should look at moving any of the messages that are not actually standard to something more like example_msgs this will set expectations more clearly.

This could have wide ranging effects and it might be worth adding explicit warnings into the generators to help with the migration process.

[tfoote]

Done in a combination of #116 and ros2/example_interfaces#11

[ros-discourse]

This issue has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/soss-a-whole-new-approach-to-your-ros-1-ros-2-bridge/17040/18