feature: expose preimage in forward+settle event

Until now, clients of SubscribeHTLCEvents didn't have access to the settled preimage. The API allows to intercept forward event and to be updated on forward events however the forward+settle event does not include the payment preimage. This pr changes allows it.

htlcswitch/interfaces.go

@@ -282,5 +282,6 @@ type htlcNotifier interface {
 	// NotifySettleEvent notifies the HtlcNotifier that a htlc that we
 	// committed to as part of a forward or a receive to our node has been
 	// settled.
-	NotifySettleEvent(key HtlcKey, eventType HtlcEventType)
+	NotifySettleEvent(key HtlcKey, preimage lntypes.Preimage,
+		eventType HtlcEventType)
 }