support for cartesian trajectories in benchmarks

This adds initial trajectory constraints support to the benchmark service. The constraints are added to the trajectory_constraints field of the MotionPlanRequest message. Then,

* When benchmarking planners, those constraints should be taken into account by the planner that must generate a path going though all of them.

* When benchmarking just kinematics, the trajectory waypoints are considered as goals. Reachability and collisions are checked for each of the waypoints.