Add callback groups to executor

[jacobperron]

Feature request

Feature description

Support adding specific callback groups to an executor, instead of entire node objects.
This enables finer control over what is added to an executor so we can have multiple executors operating on the same node.

Equivalent of ros2/rclcpp#1218, but in Python.

Implementation considerations

I'm presuming this is a feature that we'd like to have, so contradict me otherwise.

Judging by the size of the rclcpp PR, I don't expect this to be an easy enhancement to make, but figured I'd open this ticket to start a discussion.