feat: Added lazy subscribe support for GZ image bridge

[TannerGilbert]

🎉 New feature

Closes #752

Summary

Adds a lazy option to the ros_gz_image bridge similar to what already exists for the ros_gz_bridge. This can be quite important as cameras usually take a lot of resources that shouldn't be used up if there is no active subscriber.

Example command:

ros2 run ros_gz_image image_bridge /camera --ros-args -p lazy:=true

Example output:
No ROS subscribers => bridge doesn't subscribe to camera

Publishers [Address, Message Type]:
  tcp://172.17.0.1:40959, gz.msgs.Image
No subscribers on topic [/world/default/model/x500_lidar_and_camera_0/link/navigation_camera_link/sensor/navigation_camera/image]

ROS subscriber => bridge subcribes to camera:

Publishers [Address, Message Type]:
  tcp://172.17.0.1:40959, gz.msgs.Image
Subscribers [Address, Message Type]:
  tcp://172.17.0.1:37671, gz.msgs.Image

Test it

Create a world with a camera inside it. Then run:

ros2 run ros_gz_image image_bridge /camera --ros-args -p lazy:=true

If no subscriber exists gz topic -i -t /camera should return no subscriber. After subscribing with for example ros2 topic hz -w 10 /camera when reruning gz topic -i -t /camera there should be a subscriber visible.

Note: I tested on ROS2 Humble and therefore would already have a backport for this version.

Checklist

• Signed all commits for DCO
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• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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[ahcorde]

Update documentation with the new parameters

[ahcorde]

I think we don't need to keep this, it's only used in the constructor

[ahcorde]

Can we use here CheckSubscribers, the name is quite similar to rclcpp::spin which might generate some confusion

[ahcorde]

maybe 1 seconds by default is to much, 100ms?