Composition API

[crystaldust]

Make ros2cli able to handle Python components. The changes:

• Add type rclpy_components
• Get the components' metadata by py entry points, related repo: rcl_component_searchers.
• For rclpy_components, use Python's entry_points API to get the metadata, while for rclcpp_components, use a general ament resource searcher, which can also be applied to other programming language implementations(just add an entry point item in the above project's setup.py)
• When calling ros2 component list, add the component resource types(rclpy_components or rclcpp_components) to the output
• Another related example project: rclpy_composition_example

[audrow]

@crystaldust, any updates on this? I believe that this is waiting on changes to be made to https://github.com/ros2/rclpy/pull/599, correct?

[crystaldust]

@audrow Yes, this feature depends on #554 on rclpy. I'll make sure 554 is first merged, and then let's consider this one.