Add reusable two-factor verification for login and money operations (#32)

frontend/packages/sxchnge_auth/lib/src/dto/two_factor.dart

@@ -0,0 +1,25 @@
+class FactorSettingsDTO {
+  const FactorSettingsDTO(this.enabled, this.target, this.method);
+  final bool enabled;
+  final String method;
+  final String target;
+}
+
+class FactorChallengeDTO {
+  const FactorChallengeDTO(
+    this.id,
+    this.purpose,
+    this.operation,
+    this.target,
+    this.expiresAt,
+    this.resendAt,
+    this.method,
+  );
+  final String id, purpose, operation, target, method;
+  final DateTime expiresAt, resendAt;
+}
+
+class FactorGrantDTO {
+  const FactorGrantDTO(this.token);
+  final String token;
+}