Add ActionServer (#270)

* Add Action server functions to extension module
    * Separated service related macros into separate request and response calls
* Add server goal handle functions to extension module
* Update Action extension module to use conversion functions
* Add implementation of Python ActionServer
    * Handles goal and cancel requests, responds, and calls user-defined functions for executing goals.
    * Handle result requests
    * Handle expired goals
    * Publish goal status array and feedback
* Add `handle_accepted_callback` to ActionServer

Upon accepting a goal, the provided `handle_accepted_callback` is called with a reference to the goal handle.
The user can then decide to execute the goal or defer.
If `handle_accepted_callback` is not provided, then the default behavior is to execute the goal handle immediately.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

rclpy/rclpy/action/__init__.py

@@ -13,3 +13,4 @@
 # limitations under the License.
 
 from .client import ActionClient  # noqa
+from .server import ActionServer, CancelResponse, GoalResponse  # noqa