Update download system

增加证书认证方法

Assets/YooAsset/Runtime/DownloadSystem/DownloadSystem.cs

@@ -2,6 +2,7 @@
 using System.IO;
 using System.Collections;
 using System.Collections.Generic;
+using UnityEngine.Networking;
 
 namespace YooAsset
 {
@@ -15,6 +16,12 @@ namespace YooAsset
 		private static readonly Dictionary<string, DownloaderBase> _downloaderDic = new Dictionary<string, DownloaderBase>();
 		private static readonly List<string> _removeList = new List<string>(100);
 
+
+		/// <summary>
+		/// 自定义的证书认证实例
+		/// </summary>
+		public static CertificateHandler CertificateHandlerInstance;
+
 		/// <summary>
 		/// 启用断点续传功能文件的最小字节数
 		/// </summary>