[CLEAN] using function templates for multiple shm msg types.

[ZhenshengLee]

• [ ] https://github.com/ZhenshengLee/ros2_shm_msgs/blob/master/include/shm_msgs/open3d_conversions.hpp
• [ ] https://github.com/ZhenshengLee/ros2_shm_msgs/blob/master/include/shm_msgs/opencv_conversions.hpp
• [ ] https://github.com/ZhenshengLee/ros2_shm_msgs/blob/master/include/shm_msgs/point_cloud2_iterator.hpp

[ZhenshengLee]

• using total template specialization and part template specialization
• using non-type template arguments