routerrpc: expose preimage on the htlc level

lnrpc/routerrpc/router.swagger.json

@@ -214,6 +214,11 @@
         "failure": {
           "$ref": "#/definitions/lnrpcFailure",
           "description": "Detailed htlc failure info."
+        },
+        "preimage": {
+          "type": "string",
+          "format": "byte",
+          "description": "The preimage that was used to settle the HTLC."
         }
       }
     },

lnrpc/routerrpc/router_backend.go

@@ -872,6 +872,7 @@ func (r *RouterBackend) MarshalHTLCAttempt(
 		rpcAttempt.ResolveTimeNs = MarshalTimeNano(
 			htlc.Settle.SettleTime,
 		)
+		rpcAttempt.Preimage = htlc.Settle.Preimage[:]
 
 	case htlc.Failure != nil:
 		rpcAttempt.Status = lnrpc.HTLCAttempt_FAILED