Add systemd cgroup path spec

[AkihiroSuda]

https://github.com/opencontainers/runtime-spec/blob/master/config-linux.md#cgroups-path

The systemd cgroup path convention implemented in runc/crun should be added to the spec.

This convention is becoming important for cgroup v2, because rootless containers depends on systemd for cgroup delegation in most environments.

[kailun-qin]

Do we have any plan to move this forward? Thanks!

[AkihiroSuda]

Thanks for opening #1115, but I meant adding proper spec for the existing implementation of systemd cgroupsPath, e.g. user.slice:nerdctl:deadbeef https://github.com/containerd/nerdctl/blob/07c3ca23743308987aa3b26b1fb7b560804ff9a4/cmd/nerdctl/run_cgroup_linux.go#L53

I didn't mean adding new fields for systemd.