feat: read the node's listening sockets through napalm-device-types

ProxmoxDriver mixes in ListeningSocketsMixin (napalm-device-types 2.4.0)
and carries its command over the exec path, which runs as root either
way: whether pveproxy, a Ceph manager or anything else on the node listens
on an address reachable from outside it.

For netOrk#658.

tests/test_listening_sockets.py

@@ -0,0 +1,37 @@
+"""`get_listening_sockets`: what listens on the node, and which service it is.
+
+Whether pveproxy, a Ceph manager or a guest-facing service is reachable from
+outside the node is decided by the address it listens on. The command and its
+parse are napalm-device-types'; the driver only carries the command over its
+exec path, which already runs as root.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import patch
+
+from napalm_device_types import ListeningSocketsMixin
+from napalm_proxmox.driver import ProxmoxDriver
+
+WIRE = (
+    "SOCK_BEGIN\n[ss]\n"
+    'tcp LISTEN 0 4096 *:8006 *:* users:(("pveproxy worker",pid=2101,fd=6))\n'
+    "__SS_RC=0\n[cgroups]\n"
+    "2101 0::/system.slice/pveproxy.service\n"
+    "SOCK_END\n"
+)
+
+
+def test_the_driver_declares_the_contract():
+    assert issubclass(ProxmoxDriver, ListeningSocketsMixin)
+
+
+def test_it_reads_as_root_over_the_exec_path(driver):
+    with patch.object(driver, "_exec_ssh_command", return_value=WIRE) as exec_:
+        reading = driver.get_listening_sockets()
+
+    [command] = [c.args[0] for c in exec_.call_args_list]
+    assert command.startswith("sh -c '")
+    assert reading["attributed"] is True
+    [socket] = reading["sockets"]
+    assert (socket["address"], socket["port"], socket["unit"]) == ("*", 8006, "pveproxy")