Provide system modes observer / cache

[norro]

Provide a system modes observer / cache (class) that can be used inside ROS 2 nodes. The client shall

• not do the mode inference on its own but rather monitor the according topics, i.e. /{part}/transition_event and /{part}/mode_event
• start with initial service requests to obtain the state/mode of the observed systems and nodes and afterwards rely on the
• be configurable wrt. the systems and nodes to observe

[norro]

@nielsvd