Refactor Client.call() to use middlewares

Author: stnguyen90

lib/appwrite.dart

@@ -6,6 +6,8 @@ import 'dart:math';
 import 'dart:typed_data';
 
 import 'models.dart' as models;
+import 'src/call_handlers/offline_call_handler.dart';
+import 'src/call_params.dart';
 import 'src/enums.dart';
 import 'src/input_file.dart';
 import 'src/service.dart';

lib/query.dart

@@ -47,8 +47,8 @@ class Query {
   static String parseValues(dynamic value) =>
       (value is String) ? '"$value"' : '$value';
 
-  String method;
-  List<dynamic> params;
+  final String method;
+  final List<dynamic> params;
 
   factory Query.parse(String query) {
     if (!query.contains('(') || !query.contains(')')) {