multi: add IsBlinded to lnrpc.Invoice for nicer UX

The BlindedPathConfig struct is nice for invoice creation but when we
use the Invoice message for viewing an invoice, it would be nicer to see
an "is_blinded" field.

rpcserver.go

@@ -5781,7 +5781,7 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 	var (
 		defaultDelta = r.cfg.Bitcoin.TimeLockDelta
 		blindCfg     = invoice.BlindedPathConfig
-		blind        = blindCfg != nil
+		blind        = invoice.IsBlinded
 	)
 
 	globalBlindCfg := r.server.cfg.Routing.BlindedPaths
@@ -5792,7 +5792,12 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 		NodeOmissionSet:          fn.NewSet[route.Vertex](),
 	}
 
-	if blind {
+	if blindCfg != nil && !blind {
+		return nil, fmt.Errorf("blinded path config provided but " +
+			"IsBlinded not set")
+	}
+
+	if blind && blindCfg != nil {
 		if blindCfg.MinNumRealHops != nil {
 			blindingRestrictions.MinDistanceFromIntroNode =
 				uint8(*blindCfg.MinNumRealHops)