Divergence of MotionPlanRequest.msg between ROS 1 and 2 Versions

[acbuynak]

Stumbled across a breaking change between versions of msg/MotionPlanRequest.msg while building a ROS Bridge. Reading through Issues/PR's it looks like there's an intentional history. Adding this issue to highlight the difference.

Brief History:

#113, message field was added, but wrong name used.
#122, attempts to correct field names.
#130, rewrite of 122, successfully merged into master.

All ROS 2 versions appear to be 9 commits behind, critically f562cc8

I'm unfamiliar with the ROS 2 moveit ecosystem to say the appropriateness of whether it's okay for these to match. At this time, users attempting to bridge MoveIt messages are unable to do so without modifying one side or the other. Given ROS Noetic is around until 2025, it may be useful to have them match.