add fields to set cartesian end effector speed

the fields are used by a method in the move_group_interface to pass
these values to a planning_request_adapter that recomputes the
trajectories time parameterization to reach the given cartesian speed.

msg/MotionPlanRequest.msg

@@ -47,3 +47,8 @@ float64 allowed_planning_time
 float64 max_velocity_scaling_factor
 float64 max_acceleration_scaling_factor
 
+# Maximum cartesian speed for the given end effector.
+# If max_cartesian_speed <= 0 the trajectory is not modified.
+# These fields require the following planning request adapter: default_planner_request_adapters/SetMaxCartesianEndEffectorSpeed
+string cartesian_speed_end_effector_link
+float64 max_cartesian_speed # m/s