Review stretch_driver's use of executor and callback groups

[hello-binit]

Stretch Driver is currently configured to use the MultiThreadedExecutor, with a single MutuallyExclusiveCallbackGroup shared among all the callbacks. This is effectively a SingleThreadedExecutor. To solve this issue,

• Review the state of Stretch Driver's callback groups, ensuring the callbacks are running in the most efficient way possible, and that no deadlocks are possible. A great guide on the subject of ROS2 executors and callback groups can be found here.
• Confirm whether all of the callbacks are matched to a single group, or if there is a hidden group.
• Run tests to verify that callbacks that are expected to run in parallel actually do.
• Write docs that document the design decisions for this node.