MqttSyncClientDownloadFileAsync 方法

[文件引擎] 从远程服务器下载一个文件到流中，需要指定文件类别，文件名，进度报告，本地保存的文件名
To download a file from a remote server to the stream, you need to specify the file category, file name, progress report, and file name saved locally

[文件引擎] 从远程服务器下载一个文件到本地，需要指定文件类别，文件名，进度报告，本地保存的文件名
[File Engine] To download a file from a remote server to the local, you need to specify the file category, file name, progress report, and file name saved locally