Add optional KYC flow and service-backed account limits

frontend/packages/sxchnge_kyc/lib/sxchnge_kyc.dart

@@ -0,0 +1,7 @@
+export 'src/models/identity.dart';
+export 'src/models/limits.dart';
+export 'src/providers/identity.dart';
+export 'src/providers/limits.dart';
+export 'src/services/contracts.dart';
+export 'src/services/dummy.dart';
+export 'src/services/photos.dart';