Add a --start-at-departure option for dispatch_go_to_place

[mxgrey]

This PR is motivated by #467 which introduces the start_at_departure option for go_to_place events.

In this PR we add a --start-at-departure flag for dispatch_go_to_place so we can test the behavior of the new flag.

To try it out, launch the hotel world:

ros2 launch rmf_demos_gz hotel.launch.xml

Then in another terminal dispatch a go_to_place task for tinyBot_1, telling it to start at the 90s mark:

ros2 run rmf_demos_tasks dispatch_go_to_place --use_sim_time -F tinyRobot -R tinyBot_1 -p L2_master_suite -st 90

The robot should start moving to the goal right away because the default behavior is to arrive at the goal by the start time.

Now if you restart the simulation and try again with the new flag, the robot should wait at its charger until the simulation reaches its 90s mark:

ros2 run rmf_demos_tasks dispatch_go_to_place --use_sim_time -F tinyRobot -R tinyBot_1 -p L2_master_suite -st 90 --start-at-departure