Support nullable download timeout option (#98)

Changed DownloadTimeOut option type to nullable int and updated usage to handle null values, allowing more flexible configuration of download timeout settings.

Author: JusterZhu

src/c#/GeneralUpdate.ClientCore/GeneralClientBootstrap.cs

@@ -31,7 +31,7 @@ private class UpdateOptionPool : ConstantPool
         /// <summary>
         /// Timeout period (unit: second). If this parameter is not specified, the default timeout period is 30 seconds.
         /// </summary>
-        public static readonly UpdateOption<int> DownloadTimeOut = ValueOf<int>("DOWNLOADTIMEOUT");
+        public static readonly UpdateOption<int?> DownloadTimeOut = ValueOf<int?>("DOWNLOADTIMEOUT");
 
         /// <summary>
         /// Whether to enable the driver upgrade function.

src/c#/GeneralUpdate.Common/Internal/Bootstrap/UpdateOption.cs

@@ -247,8 +247,7 @@ private void ApplyRuntimeOptions()
         {
             _configInfo.Encoding = GetOption(UpdateOption.Encoding) ?? Encoding.Default;
             _configInfo.Format = GetOption(UpdateOption.Format) ?? Format.ZIP;
-            var downloadTimeoutOption = GetOption(UpdateOption.DownloadTimeOut);
-            _configInfo.DownloadTimeOut = downloadTimeoutOption ?? 60;
+            _configInfo.DownloadTimeOut = GetOption(UpdateOption.DownloadTimeOut) ?? 60;
             _configInfo.DriveEnabled = GetOption(UpdateOption.Drive) ?? false;
             _configInfo.PatchEnabled = GetOption(UpdateOption.Patch) ?? true;
         }