Migrate constants from c++ image headers into the sensor_msgs/Image as enums or consts or consts

[tfoote]

This is a follwup to the pre Foxy Message API review

common_interfaces/sensor_msgs/msg/Image.msg

Line 21 in 519e851

string encoding # Encoding of pixels -- channel meaning, ordering, size

From:

common_interfaces/sensor_msgs/include/sensor_msgs/image_encodings.hpp

Line 47 in 519e851

const char RGB8[] = "rgb8";

[tfoote]

Also make sure that we're not missing common ones that are now in OpenCV and other places.

https://github.com/ros2/common_interfaces/pull/86/files#r392504087