Add `Captcha` class and return type

lib/client/v1/main.dart

@@ -49,3 +49,12 @@ extension VoteValue on Vote {
     throw Exception("unreachable");
   }
 }
+
+class Captcha {
+  final String png;
+
+  /// can be null
+  final String wav;
+  final String uuid;
+  Captcha({this.png, this.wav, this.uuid});
+}

lib/client/v1/user_endpoint.dart

@@ -25,7 +25,7 @@ extension UserEndpoint on V1 {
 
   /// GET /user/get_captcha
   /// https://dev.lemmy.ml/docs/contributing_websocket_http_api.html#get-captcha
-  String getCaptcha() {
+  Future<Captcha> getCaptcha() {
     throw UnimplementedError();
   }