Implement _config_flags_extra for services in _common role

[MajorP93]

I'd like to request a feature that does what "prometheus_config_flags_extra" ansible variable and logic does for prometheus but for services / exporters like cAdvisor.

The feature should be implemented in _common role so it is reusable.

It would be very beneficial if additional startup arguments could be passed to "ExecStart" in the respective systemd unit for services / exporters by setting an Ansible variable which is exactly what "prometheus_config_flags_extra" is already doing for prometheus.

By having that feature users of this Ansible collection would be able to have more fine grained control and use all available start up options of services / exporters that can be deployed using this collection.