Adds systemd-related notifiable handlers

- Reload systemd daemon
  - Reloads all systemd unit files
- Apply systemd unit restarts
  - Restarts multiple systemd units in a single transaction
  - Units to be restarted are defined in the `systemd_restart_units` variable of type list
  - The variable should be set using `ansible.builtin.set_fact`, with `# noqa: var-naming[no-role-prefix]`
  - The list must not be overridden, but appended (`systemd_restart_units: "{{ systemd_restart_units + ['restartme.service'] }}"`