refactor：增加ImageUpload，改用策略模式上传图片

lib/utils/strategy/github_image_upload.dart

@@ -0,0 +1,17 @@
+import 'package:flutter_picgo/model/uploaded.dart';
+import 'dart:io';
+
+import 'package:flutter_picgo/utils/strategy/image_upload_strategy.dart';
+
+class GithubImageUpload implements ImageUploadStrategy {
+
+  @override
+  Future<Uploaded> delete(Uploaded uploaded) async {
+    return null;
+  }
+
+  @override
+  Future<Uploaded> upload(File file, String renameImage) {
+    return null;
+  }
+}