Wire through CRI ContainerCheckpoint RPC

This connects the new CRI ContainerCheckpoint RPC to the existing
internal checkpoint functions. With this commit it is possible
to checkpoint a container in Kubernetes using the Forensic Container
Checkpointing KEP (#2008):

 # curl X POST "https://localhost:10250/checkpoint/namespace/podId/container"

Which will result in containerd creating a checkpoint in the location
specified by Kubernetes (usually /var/lib/kubelet/checkpoints).

This is a Linux only feature because CRIU only exists on Linux.

Rewritten with the help of Phil Estes.

Signed-off-by: Phil Estes <estesp@gmail.com>
Signed-off-by: Adrian Reber <areber@redhat.com>

go.sum

@@ -63,6 +63,10 @@ github.com/cenkalti/backoff/v4 v4.2.1/go.mod h1:Y3VNntkOUPxTVeUxJ/G5vcM//AlwfmyY
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