feat: get_service_packages, which packages a node's systemd unit is (NetOrk/netork#1236)

Mixes in napalm-device-types' ServicePackagesMixin; the transport is the
node's exec path, as for the kernel facts. Read-only. Requires
napalm-device-types 4.20.0; version 0.6.0.

napalm_proxmox/system_mixin.py

@@ -269,6 +269,10 @@ class ProxmoxSystemMixin:
         """The transport for ``KernelFactsMixin.get_kernel_facts``: the exec path."""
         return self._exec_ssh_command(command)
 
+    def _run_service_packages_command(self, command: str) -> str:
+        """The transport for ``ServicePackagesMixin.get_service_packages``: the exec path."""
+        return str(self._exec_ssh_command(command))
+
     # ------------------------------------------------------------------ #
     # Listening sockets (ListeningSocketsMixin supplies get_listening_sockets)
     # ------------------------------------------------------------------ #