Add keep alive since clients mostly get contacted by the server

nixos/modules/monitoring/vpn/client.nix

@@ -77,6 +77,7 @@ in {
           endpoint = cfg.client.endpoint;  # meaning: the server.
           publicKey = builtins.readFile(cfg.client.endpointPublicKeyFile);
           presharedKeyFile = toString cfg.client.presharedKeyFile;
+          persistentKeepalive = 25;
         }
       ];
     };