Right now, you can only add whole nodes to an executor, but it's come up in a few scenarios where you'd rather have the ability to add selective callback groups to an executor rather than the whole node. This would allow you to do things like have a separate executor for one subscription and the main executor can handle everything else in the node. This is also more similar to how callback queues worked in ROS 1.

Some things to consider:

• currently the executor assumes it doesn't share nodes with other executors
• the guard condition that interrupts an executor when new things are added (e.g. new timer) is owned by the node, but now it would need to be owned by the callback group (potentially increasing the total number of guard conditions)
• callback groups should still only be associated with a single executor, and that logic is currently in nodes (so it will need to be moved)