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

frontend/packages/sxchnge_wallet/lib/src/destinations/service.dart

@@ -0,0 +1,39 @@
+import 'models.dart';
+
+abstract interface class DestinationService {
+  bool get isSimulated;
+  Future<DestinationCapabilities> capabilities();
+  Future<List<SavedDestination>> list();
+  Future<SavedDestination> get(String id);
+  Future<DestinationValidation> validate(DestinationDetails details);
+  Future<PayoutEligibility> assess(
+    DestinationInput input,
+    PayoutContext context,
+  );
+  Future<CreationResult> create(
+    DestinationDetails details,
+    String nickname,
+    String idempotencyKey, {
+    String? replaces,
+  });
+  Future<CreationResult?> creation(String idempotencyKey);
+  Future<SavedDestination> rename(
+    String id,
+    String nickname,
+    int metadataRevision,
+  );
+  Future<void> remove(String id, int metadataRevision, int executionRevision);
+  Future<List<Bank>> banks(String query, {Future<void>? abortTrigger});
+  Future<DestinationLease> prepare(
+    DestinationInput input,
+    PayoutContext context,
+  );
+
+  Future<SavedDestination> restoreCard(
+    String id,
+    int executionRevision,
+    CardDestination capture,
+    String idempotencyKey,
+  );
+  void clearTransient();
+}