Add Cartesian trajectories to benchmarking

[mateiciocarlie]

Add the ability to define straight-line trajectories in Cartesian space that the end-effector must follow. We would need:

• a way in the GUI to specify a start pose and an end pose for the end-effector
• a separate benchmark that attempts to plan between them with path constraints and tells us how many paths were able to be executed
• (maybe?) a way to specify other things like how strictly the path constraint has to be

We would probably use this for

• typical trajectories that occur during teleoperation, such as straight lines in the robot's workspace
• door / drawer opening trajectories

[marioprats]

The user will need to store and retrieve trajectories from the warehouse, as it is done with goals. Should we store TrajectoryConstraints messages? I guess we will need to create the corresponding MoveItMessageStorage specialized class into moveit_ros/warehouse. Does this make sense?