Add support for preemption in actions

[naiveHobo]

Currently, there's no predefined way to handle preemption in rclcpp_actions. When a client sends a new goal when a previous goal is running, the old goal's state is set to ABORTED. This means that there's no way to differentiate between an abort caused by a true failure in the server and an abort caused by a request for preemption.

A client should be able to distinguish between the result codes received on true abort and abort due to preemption.

There are some ongoing discussions on the tickets and PRs listed below: https://github.com/ros2/rclcpp/issues/1104 https://github.com/ros2/rclcpp/pull/1117 https://github.com/ros2/rcl_interfaces/pull/105 https://github.com/ros-planning/navigation2/issues/1652