use asString() for directory url

toString() isn't defined due to Display trait not yet implemented in
uniffi-dart, so use asString() instead which is defined.

Author: spacebear21

payjoin-ffi/dart/test/test_payjoin_integration_test.dart

@@ -319,7 +319,7 @@ void main() {
       var services = payjoin.TestServices.initialize();
 
       services.waitForServicesReady();
-      var directory = services.directoryUrl().toString();
+      var directory = services.directoryUrl().asString();
       var ohttp_keys = services.fetchOhttpKeys();
       var ohttp_relay = services.ohttpRelayUrl();
       var agent = http.Client();