Add progress of async operation

异步操作类增加进度查询。

Assets/YooAsset/Runtime/DownloadSystem/UnityWebDataRequester.cs

@@ -83,6 +83,16 @@ namespace YooAsset
 			return _operationHandle.isDone;
 		}
 
+		/// <summary>
+		/// 下载进度
+		/// </summary>
+		public float Progress()
+		{
+			if (_operationHandle == null)
+				return 0;
+			return _operationHandle.progress;
+		}
+
 		/// <summary>
 		/// 下载是否发生错误
 		/// </summary>