Feature request: a helper for getting an absolute path to the cgroup of a container and a pod

Currently NRI plugins can ask the cgroup path of a container using `ctr.GetCgroupsPath()`.

If a plugin needs to access controllers in the cgroup, it will need to implement the magic that converts this relative path into an absolute path in the system. Unfortunately this task is quite prone to errors, because the directory structure may vary quite a bit, depending on the cgroup manager, pod QoS class (when using Kubernetes), and of course custom cgroup hierarchies.

In order to help writing plugins that work everywhere, could the NRI framework provide a way to get an absolute path to the cgroup v2 directory of a pod and of a container? `ctr.GetCgroup2AbsPath()` or something similar?