[#1243] node/cfg: Add notification config

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

cmd/neofs-node/config.go

@@ -97,6 +97,8 @@ type cfg struct {
 
 	cfgObject cfgObject
 
+	cfgNotifications cfgNotifications
+
 	metricsCollector *metrics.StorageMetrics
 
 	workers []worker
@@ -183,6 +185,12 @@ type cfgObject struct {
 	cfgLocalStorage cfgLocalStorage
 }
 
+type cfgNotifications struct {
+	enabled      bool
+	nw           notificationWriter
+	defaultTopic string
+}
+
 type cfgLocalStorage struct {
 	localStorage *engine.StorageEngine