Add X-mode for debugging minimal systems

[kvaps]

X-mode

X-mode can be useful for debugging minimal systems that do not have a built-in shell (eg. Talos).
Here's an example of how you can debug the network for a rootless kube-apiserver container without a filesystem:

kubectl node-shell -x <node>

# Download crictl
wget https://github.com/kubernetes-sigs/cri-tools/releases/download/v1.28.0/crictl-v1.28.0-linux-amd64.tar.gz -O- | \
  tar -xzf- -C /usr/local/bin/

# Setup CRI endpoint
export CONTAINER_RUNTIME_ENDPOINT=unix:///host/run/containerd/containerd.sock

# Find your container
crictl ps | grep kube-apiserver
#3ff4626a9f10e       e7972205b6614       6 hours ago         Running             kube-apiserver         0                   215107b47bd7e       kube-apiserver-talos-rzq-nkg

# Find pid of the container
crictl inspect 3ff4626a9f10e | grep pid
#    "pid": 2152,
#            "pid": 1
#            "type": "pid"
#                "getpid",
#                "getppid",
#                "pidfd_open",
#                "pidfd_send_signal",
#                "waitpid",

# Go to network namespace of the pid, but keep mount namespace of the debug container
nsenter -t 2152 -n