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/DefaultWebRemoteFileSystem/DefaultWebRemoteFileSystem.cs

@@ -10,6 +10,11 @@ namespace YooAsset
     /// </summary>
     internal class DefaultWebRemoteFileSystem : IFileSystem
     {
+        /// <summary>
+        /// 下载后台接口
+        /// </summary>
+        public IDownloadBackend DownloadBackend { private set; get; }
+
         /// <summary>
         /// 包裹名称
         /// </summary>
@@ -128,6 +133,10 @@ namespace YooAsset
         public virtual void OnCreate(string packageName, string packageRoot)
         {
             PackageName = packageName;
+
+            // 创建默认的下载后台接口
+            if (DownloadBackend == null)
+                DownloadBackend = new UnityWebRequestBackend(DownloadSystemHelper.UnityWebRequestCreater);
         }
         public virtual void OnDestroy()
         {