sched_ext: Introduce cgroup_lifetime_notifier

sched_ext: Introduce cgroup_lifetime_notifier

Other subsystems may make use of the cgroup hierarchy with the cgroup_bpf
support being one such example. For such a feature, it's useful to be able
to hook into cgroup creation and destruction paths to perform
feature-specific initializations and cleanups.

Add cgroup_lifetime_notifier which generates CGROUP_LIFETIME_ONLINE and
CGROUP_LIFETIME_OFFLINE events whenever cgroups are created and destroyed,
respectively.

The next patch will convert cgroup_bpf to use the new notifier and other
uses are planned.

Signed-off-by: Tejun Heo <tj@kernel.org>