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/htlcnotifier.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
+	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/subscribe"
 )
@@ -281,6 +282,9 @@ type SettleEvent struct {
 	// forwards with their corresponding forwarding event.
 	HtlcKey
 
+	// Preimage that was released for settling the htlc.
+	Preimage lntypes.Preimage
+
 	// HtlcEventType classifies the event as part of a local send or
 	// receive, or as part of a forward.
 	HtlcEventType
@@ -360,9 +364,12 @@ func (h *HtlcNotifier) NotifyForwardingFailEvent(key HtlcKey,
 // to as part of a forward or a receive to our node has been settled.
 //
 // Note this is part of the htlcNotifier interface.
-func (h *HtlcNotifier) NotifySettleEvent(key HtlcKey, eventType HtlcEventType) {
+func (h *HtlcNotifier) NotifySettleEvent(key HtlcKey,
+	preimage lntypes.Preimage, eventType HtlcEventType) {
+
 	event := &SettleEvent{
 		HtlcKey:       key,
+		Preimage:      preimage,
 		HtlcEventType: eventType,
 		Timestamp:     h.now(),
 	}