Add saved payout destination UX and service contracts (#54)

frontend/packages/sxchnge_wallet/lib/src/providers/wallet.dart

@@ -6,12 +6,19 @@ import 'package:sxchnge_analytics/sxchnge_analytics.dart';
 
 import 'package:sxchnge_auth/sxchnge_auth.dart';
 
+import '../destinations/service.dart';
 import '../models/wallet.dart';
 import '../services/contracts.dart';
 
 class WalletProvider extends ChangeNotifier {
-  WalletProvider(this.service, this.analytics, {this.factors});
+  WalletProvider(
+    this.service,
+    this.analytics, {
+    this.factors,
+    this.destinations,
+  });
   final WalletService service;
+  final DestinationService? destinations;
   final TwoFactorService? factors;
   final AnalyticsService analytics;
   WalletSnapshot? data;