Feature: Add CompressedImage publisher

[Aposhian]

Feature description

I want to be able to run

ros2 run compressed_image_publisher compressed_image_publisher <some-compressed-stream-url>

and see compressed images published as sensor_msgs/CompressedImage messages. I want a way to bring compressed images into ROS without doing any decompression or recompression.

Rationale

image_publisher uses cv::VideoCapture to open arbitrary video streams, and then decodes them into raw frames and publishes as sensor_msgs/Image. However, it is sometimes desirable to use sensor_msgs/CompressedImage message for either storage or network bandwidth concerns.

It is currently possible to use image_publisher to get a raw Image, and then use compressed_image_transport to recompress the image, but that is quite inefficient.

Implementation Concerns

While there is already support in cv_bridge for building CompressedImage messages from cv::Mat, I can't see a way to use cv::VideoCapture in a way that does not decode frames.

Also, would this better belong in image_pipeline or image_transport_plugins?