Feature request

Feature description

Add support for ROS Actions as described in ros2/design#193.

Includes:

• Action client Add Action Client #262
• Action server Add ActionServer #270

Metaticket to ros2/ros2#583
Requires ros2/rosidl_python#14

Implementation considerations

We'd like to add this feature to Crystal, so we should avoid disruptions to the existing code (if possible).
Isolating implementation to a new rclpy.action module would be ideal.

Usage examples can be found in ros2/examples#222 (subject to change).