lnrpc: set a zero value endorsement signal on sender outgoing htlc

Before we have sufficient signaling in the network to relay this
signal, set a zero value experimental endorsement value on the sender's
outgoing htlc. Once the network is relaying this signal and a flag day
has been set, we'll be able to set a non-zero value here.

lnrpc/routerrpc/router_backend.go

@@ -110,6 +110,10 @@ type RouterBackend struct {
 	// ParseCustomChannelData is a function that can be used to parse custom
 	// channel data from the first hop of a route.
 	ParseCustomChannelData func(message proto.Message) error
+
+	// ShouldSetExpEndorsement returns a boolean indicating whether the
+	// experimental endorsement bit should be set.
+	ShouldSetExpEndorsement func() bool
 }
 
 // MissionControl defines the mission control dependencies of routerrpc.
@@ -891,6 +895,23 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 	}
 	payIntent.FirstHopCustomRecords = firstHopRecords
 
+	// If the experimental endorsement signal is not already set, propagate
+	// a zero value field if configured to set this signal.
+	if r.ShouldSetExpEndorsement() {
+		if payIntent.FirstHopCustomRecords == nil {
+			payIntent.FirstHopCustomRecords = make(
+				map[uint64][]byte,
+			)
+		}
+
+		t := uint64(lnwire.ExperimentalEndorsementType)
+		if _, set := payIntent.FirstHopCustomRecords[t]; !set {
+			payIntent.FirstHopCustomRecords[t] = []byte{
+				lnwire.ExperimentalUnendorsed,
+			}
+		}
+	}
+
 	payIntent.PayAttemptTimeout = time.Second *
 		time.Duration(rpcPayReq.TimeoutSeconds)