nav2_way_point_follower; introduce photo at waypoint arrivals plugin (#2041)

* nav2_way_point_follower; introduce photo at waypoint arrivals plugin

Signed-off-by: jediofgever <fetulahatas1@gmail.com>

* resolve cmake lint errors

Signed-off-by: jediofgever <fetulahatas1@gmail.com>

* resolve requested chages of first review

Signed-off-by: jediofgever <fetulahatas1@gmail.com>

* minor corrections on photo_at_waypoint header

* resolve requested changes of second review

Signed-off-by: jediofgever <fetulahatas1@gmail.com>

* update default save_dir

Signed-off-by: jediofgever <fetulahatas1@gmail.com>

* move directory checking code to initialize()