refactor the runtime code

重构了运行时代码，支持全新的文件系统。

Assets/YooAsset/Runtime/FileSystem/Operation/Internal/FSLoadBundleFileOperation.cs

@@ -0,0 +1,27 @@
+using UnityEngine;
+
+namespace YooAsset
+{
+    internal abstract class FSLoadBundleOperation : AsyncOperationBase
+    {
+        /// <summary>
+        /// 加载结果
+        /// </summary>
+        public object Result { protected set; get; }
+
+        /// <summary>
+        /// 下载进度
+        /// </summary>
+        public float DownloadProgress { protected set; get; }
+
+        /// <summary>
+        /// 下载大小
+        /// </summary>
+        public long DownloadedBytes { protected set; get; }
+
+        /// <summary>
+        /// 终止下载任务
+        /// </summary>
+        public abstract void AbortDownloadOperation();
+    }
+}