Make publishers context-manager aware.

[clalancette]

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.

[clalancette]

I should mention that this partially fixes https://github.com/ros2/rclpy/issues/1280 , though we'll need a bunch of other PRs before it is completely done.

[ahcorde]

• Linux
• Linux-aarch64
• Windows