runk: implement the unimplemented features

[liubin]

@ManaSugi has created runk as a container runtime, it works only with some sub-commands that are not unimplemented.

This issue is used to track these unimplemented sub-commands.

basic features

• runk: support ps sub-command runk: support ps sub-command #4361
• runk: support list sub-command runk: support list sub-command #4362
• runk: support exec sub-command runk: support exec sub-command #4363
• runk: support Systemd Cgroups runk: support Systemd Cgroups #4369
• runk: support update sub-command runk: support update sub-command #4365
• runk: support Rootless runk: support Rootless #4372
• docs: add to runtime spec implementations

enhancement features

• runk: support pause/resume sub-command runk: support pause/resume sub-command #4364
• runk: support events sub-command runk: support events sub-command #4366
• runk: support init sub-command runk: support init sub-command #4367
• runk: support checkpoint/restore sub-command runk: support checkpoint/restore sub-command #4368
• runk: support pivot_root runk: support pivot_root #4370
• runk: support SELinux runk: support SELinux #4371

[rata]

The "features" sub-command will be nice to support to.

It is becoming increasingly important for user namespace adoption to see if the OCI runtime support idmap mounts (via the features subcommand we check that). For more details, see here: opencontainers/runtime-spec#1219. We are enforcing the OCI runtime supports idmap on containerd and crio for userns support.