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.

lnrpc/routerrpc/router.proto

@@ -607,6 +607,8 @@ message ForwardFailEvent {
 }
 
 message SettleEvent {
+    // The revealed preimage.
+    bytes preimage = 1;
 }
 
 message LinkFailEvent {