[ros2] re-introduce addDiagnostics to aggregator

[norro]

During migration of the diagnostics aggregator to ROS 2, the addDiagnostics feature was kicked out. This was mainly done due to its dependency on bond.

Former documentation of this feature was: "Service request callback for addition of diagnostics. Creates a bond between the calling node and the aggregator, and loads information about new diagnostics into added_analyzers_, keeping track of the formed bond in bonds_."

Re-introduce this feature, but check if heartbeat can be handled with rmw mechanisms instead of bond.

[Karsten1987]

looking at RMW, I think the liveliness assertion might be a good entry point for this:
https://github.com/ros2/rmw/blob/master/rmw/include/rmw/rmw.h#L490-L512

Not sure how well supported it is by all RMWs, but we could think of making it optional.