Make publishers context-manager aware. (#1289)

The main reason to do this is so that we can change example
code to use context managers for managing the lifetimes
of publishers (and other entities), which should make the
code significantly easier to read.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

rclpy/rclpy/publisher.py

@@ -12,7 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Generic, List, Type, TypeVar, Union
+from types import TracebackType
+from typing import Generic, List, Optional, Type, TypeVar, Union
 
 from rclpy.callback_groups import CallbackGroup
 from rclpy.duration import Duration
@@ -128,3 +129,14 @@ def wait_for_all_acked(self, timeout: Duration = Duration(seconds=-1)) -> bool:
         """
         with self.handle:
             return self.__publisher.wait_for_all_acked(timeout._duration_handle)
+
+    def __enter__(self) -> 'Publisher':
+        return self
+
+    def __exit__(
+        self,
+        exc_type: Optional[Type[BaseException]],
+        exc_val: Optional[BaseException],
+        exc_tb: Optional[TracebackType],
+    ) -> None:
+        self.destroy()