Develop Concept Architecture for integration of new Sentor statemachine with ROS2.

[marc-hanheide]

Following the merge of #62 and #63 it is now time to consider how the interaction between sentor, the new https://github.com/LCAS/RobotStateMachine implementation and an implementation of an autonomous navigation stack, implemented along the guidelines of Nav2 (https://docs.nav2.org/) should work.

The core idea is that the state machine developed in https://github.com/LCAS/RobotStateMachine/blob/main/src/robot_state_machine/robot_state_machine/robot_state_machine.py determines and constraints the behaviour of the autonomous navigation, i.e., that the robot only ever moves under the following conditions:

/robot_state == "active" and /autonomous_mode == true

Whenever either a state or mode change invalidates the condition, the robot must stop immediately, and any nav2 navigation must stop (and reported back as interrupted or failed).

This issue is just to draft a plan for an implementation of this interaction. It is need to create any code. Only make proposals for how this could be best implemented to be compatible with a nav2 stack, ensuring safe and compliant autonomy