Triggered cameras (take photos instead of videos)

[chapulina]

Gazebo classic supports cameras that only capture images when triggered through ROS, see gazebo_ros_camera. When porting that to Ignition, we should make all cameras "triggerable" via an Ignition topic that can be bridged to a ROS topic with ros_ign.

Desired behavior

Cameras that have <triggered>true</triggered> will only capture an image when they receive a command on an Ignition topic (like taking a photo instead of recording a video).

Alternatives considered

Users may be able to achieve this with a custom plugin, but adding this functionality to all cameras by default should be easier to maintain and extend. We may also consider adding this functionality to all sensors? I can see this being useful for performance when using lidars, contact sensors, etc.

Implementation suggestion

1. Add a <triggered> parameter to <sensor> on SDF. Or just to <camera>.
2. Parse that tag in ign-sensors and provide the necessary topic and callbacks that enable triggering

[mjcarroll]

It would also be beneficial to support the PolledCamera interface from ROS: https://github.com/ros-perception/image_common/tree/noetic-devel/polled_camera

[chapulina]

It would also be beneficial to support the PolledCamera interface from ROS

Nice! I think that could be added to ros_ign_image.