refactor : 重构网络下载模块

新增通用下载接口，扩展了默认的Unity引擎下载器

Assets/YooAsset/Runtime/FileSystem/DefaultCacheFileSystem/DefaultCacheFileSystem.cs

@@ -23,10 +23,14 @@ namespace YooAsset
         protected string _cacheManifestFilesRoot;
 
         /// <summary>
-        /// 下载中心
-        /// 说明：当异步操作任务终止的时候，所有下载子任务都会一同被终止！
+        /// 下载调度器
         /// </summary>
-        public DownloadCenterOperation DownloadCenter { set; get; }
+        public DownloadSchedulerOperation DownloadScheduler { set; get; }
+
+        /// <summary>
+        /// 下载后台接口
+        /// </summary>
+        public IDownloadBackend DownloadBackend { private set; get; }
 
         /// <summary>
         /// 包裹名称
@@ -105,7 +109,7 @@ namespace YooAsset
         /// <summary>
         /// 自定义参数：下载任务的看门狗机制监控时间
         /// </summary>
-        public int DownloadWatchDogTime { private set; get; } = int.MaxValue;
+        public int DownloadWatchDogTime { private set; get; } = 0;
 
         /// <summary>
         /// 自定义参数：启用断点续传的最小尺寸
@@ -288,7 +292,7 @@ namespace YooAsset
             else if (name == FileSystemParametersDefine.DOWNLOAD_WATCH_DOG_TIME)
             {
                 int convertValue = Convert.ToInt32(value);
-                DownloadWatchDogTime = Mathf.Clamp(convertValue, 1, int.MaxValue);
+                DownloadWatchDogTime = Mathf.Clamp(convertValue, 0, int.MaxValue);
             }
             else if (name == FileSystemParametersDefine.RESUME_DOWNLOAD_MINMUM_SIZE)
             {
@@ -327,13 +331,17 @@ namespace YooAsset
             _cacheBundleFilesRoot = PathUtility.Combine(_packageRoot, DefaultCacheFileSystemDefine.BundleFilesFolderName);
             _cacheManifestFilesRoot = PathUtility.Combine(_packageRoot, DefaultCacheFileSystemDefine.ManifestFilesFolderName);
             _tempFilesRoot = PathUtility.Combine(_packageRoot, DefaultCacheFileSystemDefine.TempFilesFolderName);
+
+            // 创建默认的下载后台接口
+            if (DownloadBackend == null)
+                DownloadBackend = new UnityWebRequestBackend(DownloadSystemHelper.UnityWebRequestCreater);
         }
         public virtual void OnDestroy()
         {
-            if (DownloadCenter != null)
+            if (DownloadScheduler != null)
             {
-                DownloadCenter.AbortOperation();
-                DownloadCenter = null;
+                DownloadScheduler.Dispose();
+                DownloadScheduler = null;
             }
         }