Adding coverage tests for `context` module (#1321), we have realized there is no `const` method for the `shutdown_reason` method. See [link to the code](https://github.com/ros2/rclcpp/blob/3a4ac0ca2093d12035070443692798b0c9f9da3a/rclcpp/include/rclcpp/context.hpp#L153 ) This issue is to track the conversation [here](https://github.com/ros2/rclcpp/pull/1321#discussion_r491231948) and get some feedback.