multi: add blinded route to route requests expressed as hints

Add the option to include a blinded route in a route request (exclusive
to including hop hints, because it's incongruous to include both), and
express the route as a chain of hop hints.

Using a chain of hints over a single hint to represent the whole path
allows us to re-use our route construction to fill in a lot of the
path on our behalf.

lnrpc/routerrpc/router_backend.go

@@ -327,7 +327,7 @@ func (r *RouterBackend) QueryRoutes(ctx context.Context,
 	// the route.
 	routeReq, err := routing.NewRouteRequest(
 		sourcePubKey, &targetPubKey, amt, in.TimePref, restrictions,
-		customRecords, routeHintEdges, finalCLTVDelta,
+		customRecords, routeHintEdges, nil, finalCLTVDelta,
 	)
 	if err != nil {
 		return nil, err

lnrpc/routerrpc/router_server.go

@@ -404,7 +404,7 @@ func (s *Server) EstimateRouteFee(ctx context.Context,
 			FeeLimit:          feeLimit,
 			CltvLimit:         s.cfg.RouterBackend.MaxTotalTimelock,
 			ProbabilitySource: mc.GetProbability,
-		}, nil, nil, s.cfg.RouterBackend.DefaultFinalCltvDelta,
+		}, nil, nil, nil, s.cfg.RouterBackend.DefaultFinalCltvDelta,
 	)
 	if err != nil {
 		return nil, err