Compile problems with ROS2

[sssarana]

Hello,
When building for ROS2 from both ros2 and rolling branch, I faced several issues with header includes and timestamp usage. See below.

/home/ros2_ws/src/grid_map/grid_map_cv/include/grid_map_cv/GridMapCvProcessing.hpp:15:10: fatal error: cv_bridge/cv_bridge.hpp: No such file or directory
   15 | #include <cv_bridge/cv_bridge.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~
/home/ros2_ws/src/grid_map/grid_map_cv/include/grid_map_cv/GridMapCvConverter.hpp:15:10: fatal error: cv_bridge/cv_bridge.hpp: No such file or directory
   15 | #include <cv_bridge/cv_bridge.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~

Finished <<< grid_map_costmap_2d [0.15s]                                                               
--- stderr: grid_map_cv                             
/home/ros2_ws/src/grid_map/grid_map_cv/test/GridMapCvTest.cpp:13:10: fatal error: cv_bridge/cv_bridge.hpp: No such file or directory
   13 | #include <cv_bridge/cv_bridge.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
/home/ros2_ws/src/grid_map/grid_map_cv/test/GridMapCvProcessingTest.cpp:12:10: fatal error: cv_bridge/cv_bridge.hpp: No such file or directory
   12 | #include <cv_bridge/cv_bridge.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~


/home/ros2_ws/src/grid_map/grid_map_demos/src/opencv_demo_node.cpp:4:10: fatal error: cv_bridge/cv_bridge.hpp: No such file or directory
    4 | #include <cv_bridge/cv_bridge.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~

--- stderr: grid_map_ros                              
In file included from /home/ros2_ws/src/grid_map/grid_map_ros/src/GridMapRosConverter.cpp:34:
/home/ros2_ws/src/grid_map/grid_map_ros/include/grid_map_ros/GridMapRosConverter.hpp:26:10: fatal error: cv_bridge/cv_bridge.hpp: No such file or directory
   26 | #include <cv_bridge/cv_bridge.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~

Starting >>> grid_map_ros
--- stderr: grid_map_ros                             
/home/ros2_ws/src/grid_map/grid_map_ros/src/GridMapRosConverter.cpp: In static member function ‘static bool grid_map::GridMapRosConverter::saveToBag(const grid_map::GridMap&, const string&, const string&)’:
/home/ros2_ws/src/grid_map/grid_map_ros/src/GridMapRosConverter.cpp:691:52: error: ‘using element_type = struct rosbag2_storage::SerializedBagMessage’ {aka ‘struct rosbag2_storage::SerializedBagMessage’} has no member named ‘send_timestamp’
  691 |   auto ret = rcutils_system_time_now(&bag_message->send_timestamp);
      |                                                    ^~~~~~~~~~~~~~


--- stderr: grid_map_ros                             
/home/ros2_ws/src/grid_map/grid_map_ros/test/GridMapRosTest.cpp:17:10: fatal error: cv_bridge/cv_bridge.hpp: No such file or directory
   17 | #include <cv_bridge/cv_bridge.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~

I already created a pull request which fixes all these problems and adds some updates to CMake files: #520
I would appreciate it if you could review and merge.
Thank you.