Consider using systemd for pause PID

We added support for handling namespaces in https://github.com/containers/conmon-rs/pull/908, where @cgwalters mentioned that we could consider using systemd for the PID handling: https://github.com/containers/conmon-rs/pull/908#discussion_r1088243583

We can experiment with the implementation to see how much the resource overhead would be when using systemd.

Review pull request 908 and the linked discussion about namespace and pause-PID handling first. Then investigate a systemd-based implementation and measure its resource overhead against the current approach; done means the experiment has a documented result and a clearly scoped implementation or conclusion.

Written by the indexing model from the issue text.