multi: add wtclient config defaults to lnd config

In this commit, a `DefaultWtClientCfg` function is added which
populates some default values for the `WtClient` struct. This is then
used to populate the wtclient defaults in the main LND config struct.

config.go

@@ -713,6 +713,7 @@ func DefaultConfig() Config {
 			ServerPingTimeout: defaultGrpcServerPingTimeout,
 			ClientPingMinWait: defaultGrpcClientPingMinWait,
 		},
+		WtClient: lncfg.DefaultWtClientCfg(),
 	}
 }