Support ROS 2 Actions

[sea-bass]

Feature Request

As a ROS 2 developer, I want roslibpy to support ROS 2 actions with the new support in the ROSBridge protocol.

Details

In RobotWebTools/rosbridge_suite#886, we merged native ROS 2 action support to rosbridge_suite, and in RobotWebTools/roslibjs#645 we used it in roslibjs.

Creating this issue to track the work to do the same here in roslibpy, as I have already personally heard from 2 people in the past week trying to get this working and running into unexpected error with the current interface that only works for ROS 1 at the moment.