feat: get_system_identity for matching a node to its BMC (0.2.0)

Mixes in napalm-device-types 4.9's SystemIdentityMixin over the exec
path, which runs as root: the node's SMBIOS system UUID, product serial
and board serial, vendor placeholders empty. pve-dual, a Supermicro
X10DRL-i, reports the product serial 0123456789; its board serial and
UUID are what tie it to its BMC.

napalm_proxmox/system_mixin.py

@@ -235,6 +235,11 @@ class ProxmoxSystemMixin:
     # Listening sockets (ListeningSocketsMixin supplies get_listening_sockets)
     # ------------------------------------------------------------------ #
 
+    def _run_system_identity_command(self, command: str, *, privileged: bool) -> str:
+        """The transport for ``SystemIdentityMixin.get_system_identity``: the exec path,
+        which runs as root either way."""
+        return str(self._exec_ssh_command(command))
+
     def _run_listening_sockets_command(self, command: str, *, privileged: bool) -> str:
         """The transport for ``ListeningSocketsMixin.get_listening_sockets``:
         the exec path, which runs as root either way."""