dos: Move subsystem to the subsys manager
In order to be able to export MetricsPort data from the DoS subsystem (in part with #40259), we need the DoS subsystem to be added to the subsys manager so the get_metrics() method can be used.
Furthermore, to do it right, we need to move all configuration options within the module as well so we have better modularization.
This ticket is for this work.