Consolidate RobotTrajectory::getRobotTrajectoryMessage() API

[henningkayser]

I'm adding support for multi-dof to joint trajectory conversion in #2740, which initially is implemented as a free function variant of RobotTrajectory::getRobotTrajectoryMessage(). I would like to consolidate the free function with the class API (e.g. calling the function from getRobotTrajectoryMessage()), but there are still a couple shortcomings that I find should be cleaned up first, and that are out of scope of mdof trajectory support:

• Improved error checking
• simpler loop implementation relying more on name mapping API of RobotState instead of the weird dof size checking https://github.com/ros-planning/moveit2/pull/2740/files#diff-df2eb215a3ed0921e1e470c6ff61f327eecd07749531d0a0fd69065d5eedbd02R787
• handle mdof option flag

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This issue was closed because it has been stalled for 45 days with no activity.