routing: include route blinding fields in blinded portion of path

This commit updates route construction to backfill the fields
required for payment to blinded paths and set amount to forward
and expiry fields to zero for intermediate hops (as is instructed
in the route blinding specification).

We could attempt to do this in the first pass, but that loop
relies on fields like amount to forward and expiry to calculate
each hop backwards, so we keep it simple (stupid) and post
processes the blinded portion, since it's computationally cheap
and more readable.

routing/router.go

@@ -1991,6 +1991,16 @@ func getTargetNode(target *route.Vertex, blindedPayment *BlindedPayment) (
 	}
 }
 
+// blindedPath returns the request's blinded path, which is set if the payment
+// is to a blinded route.
+func (r *RouteRequest) blindedPath() *sphinx.BlindedPath {
+	if r.BlindedPayment == nil {
+		return nil
+	}
+
+	return r.BlindedPayment.BlindedPath
+}
+
 // FindRoute attempts to query the ChannelRouter for the optimum path to a
 // particular target destination to which it is able to send `amt` after
 // factoring in channel capacities and cumulative fees along the route.
@@ -2047,7 +2057,7 @@ func (r *ChannelRouter) FindRoute(req *RouteRequest) (*route.Route, float64,
 			totalAmt:  req.Amount,
 			cltvDelta: req.FinalExpiry,
 			records:   req.CustomRecords,
-		},
+		}, req.blindedPath(),
 	)
 	if err != nil {
 		return nil, 0, err
@@ -3032,7 +3042,7 @@ func (r *ChannelRouter) BuildRoute(amt *lnwire.MilliSatoshi,
 			cltvDelta:   uint16(finalCltvDelta),
 			records:     nil,
 			paymentAddr: payAddr,
-		},
+		}, nil,
 	)
 }