api: Introduce a new API to find the cgroup setup mode

api: Introduce a new API to find the cgroup setup mode

This patch introduces a new API to detect the current cgroup setup
mode (Legacy/Unified/Hybrid). The setup will depend on the Linux
Kernel's grub command line, the system/VM is booted with.

Uses cases:
Depending upon the cgroup setup, the users can enable or disable
features in their applications. Like some controllers are only available
on cgroup v2, so they might need to set/get settings for those available
controllers only.

Suggested-by: Tom Hromatka <tom.hromatka@oracle.com>
Suggested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>