custom plugin to monitor iptables versions rules

iptables has two kernel backends, legacy and nft.

Quoting https://developers.redhat.com/blog/2020/08/18/iptables-the-two-variants-and-their-relationship-with-nftables

> It is also important to note that while iptables-nft
> can supplant iptables-legacy, you should never use them simultaneously.

However, we don't want to block the node operations because of this
reason, as there is no enough evidence this is causing big issues in the
wild, so we just signal and warn about this situation.

Once we have more information we can revisit this decision and
keep it as is or move it to permanent.

config/iptables-mode-monitor.json

@@ -0,0 +1,20 @@
+{
+  "plugin": "custom",
+  "pluginConfig": {
+    "invoke_interval": "86400s",
+    "timeout": "5s",
+    "max_output_length": 80,
+    "concurrency": 1
+  },
+  "source": "iptables-mode-monitor",
+  "metricsReporting": true,
+  "conditions": [],
+  "rules": [
+    {
+      "type": "temporary",
+      "reason": "IPTablesVersionsMismatch",
+      "path": "./config/plugin/iptables_mode.sh",
+      "timeout": "5s"
+    }
+  ]
+}

config/plugin/iptables_mode.sh

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+#!/bin/bash
+
+# As of iptables 1.8, the iptables command line clients come in two different versions/modes: "legacy",
+# which uses the kernel iptables API just like iptables 1.6 and earlier did, and "nft", which translates
+# the iptables command-line API into the kernel nftables API.
+# Because they connect to two different subsystems in the kernel, you cannot mix rules from different versions.
+# Ref: https://github.com/kubernetes-sigs/iptables-wrappers
+
+readonly OK=0
+readonly NONOK=1
+readonly UNKNOWN=2
+
+# based on: https://github.com/kubernetes-sigs/iptables-wrappers/blob/97b01f43a8e8db07840fc4b95e833a37c0d36b12/iptables-wrapper-installer.sh
+readonly num_legacy_lines=$( (iptables-legacy-save || true; ip6tables-legacy-save || true) 2>/dev/null | grep -c '^-' || true)
+readonly num_nft_lines=$( (timeout 5 sh -c "iptables-nft-save; ip6tables-nft-save" || true) 2>/dev/null | grep -c '^-' || true)
+
+
+if [ "$num_legacy_lines" -gt 0 ] && [ "$num_nft_lines" -gt 0 ]; then
+  echo "Found rules from both versions, iptables-legacy: ${num_legacy_lines} iptables-nft: ${num_nft_lines}"
+  echo $NONOK
+elif [ "$num_legacy_lines" -gt 0 ] && [ "$num_nft_lines" -eq 0 ]; then
+  echo "Using iptables-legacy: ${num_legacy_lines} rules"
+  echo $OK
+elif [ "$num_legacy_lines" -eq 0 ] && [ "$num_nft_lines" -gt 0 ]; then
+  echo "Using iptables-nft: ${num_nft_lines} rules"
+  echo $OK
+else
+  echo "No iptables rules found"
+  echo $UNKNOWN
+fi