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

frontend/app/test/destination_helpers.dart

@@ -0,0 +1,31 @@
+import 'package:flutter/material.dart';
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:sxchnge_wallet/sxchnge_wallet.dart';
+
+const testDestination = CryptoDestination(
+  assetId: 'demo:ethereum:native',
+  networkId: 'eip155:1',
+  address: '0x1234567890123456789012345678901234567890',
+  assetLabel: 'ETH',
+  networkLabel: 'Ethereum',
+);
+Future<void> seedDestination(DummyDestinationService service) async {
+  await service.create(
+    testDestination,
+    'Test wallet',
+    'test-destination-creation',
+  );
+}
+
+Future<void> selectPayout(WidgetTester t) async {
+  final button = find.byKey(const ValueKey('choose-payout-destination'));
+  await t.ensureVisible(button);
+  await t.tap(button);
+  await t.pumpAndSettle();
+  final row = find.text('Test wallet');
+  await t.ensureVisible(row);
+  await t.tap(row);
+  await t.pumpAndSettle();
+}