KEP 5823: Pod-Level Checkpoint/Restore for gVisor

### Problem
KEP-5823 introduces native Pod-level checkpoint and restore in Kubernetes, adding CheckpointPod and RestorePod RPCs to the CRI RuntimeService. This shifts checkpointing from per-container states to taking a consistent snapshot of the entire sandbox/Pod boundary.

Because gVisor manages its own Sentry kernel and application state, gVisor's native [checkpoint/restore](https://gvisor.dev/docs/user_guide/checkpoint_restore/) model naturally aligns with Pod-level snapshotting much better than legacy per-container approaches (KEP-2008).

The gVisor community will partner with containerd community and other sandbox runtime for the support.

### KEP/SIG-Node References
KEP(s): KEP-5823
stage: alpha
KEP Issue: https://github.com/kubernetes/enhancements/issues/5823
KEP PR: https://github.com/kubernetes/enhancements/pull/5851
K8s-Release: v1.37 (target for Alpha)
KEP-Owner: @rst0git @viktoriaas @adrianreber @andreyvelich
SIG-Node member liason: @haircommander
KEP-Shepherd: TBD

### Containerd References
* https://github.com/containerd/containerd/issues/13979

### Kata References
* https://github.com/kata-containers/kata-containers/issues/13653