`Manager` is a trait to unify the interface of cgroups. It is designed for
OCI containers. Its `set()` takes Linux resources of the OCI spec to set
cgroups.
The `FsManager`, the concrete implementation of `Manager`, manipulates
cgroups through cgroupfs, and supports both cgroups v1 and v2.
Signed-off-by: Xuewei Niu <niuxuewei.nxw@antgroup.com>