MqttRpcDeviceReadStringAsync 方法

异步读取字符串数据，默认为最常见的ASCII编码
Asynchronously read string data, default is the most common ASCII encoding

异步使用指定的编码，读取字符串数据
Asynchronously reads string data using the specified encoding

从MQTT服务器同步读取数据，将指定编码的字符串payload发送到服务器，然后从服务器返回相关的数据，并转换为指定编码的字符串，支持数据发送进度报告，服务器执行进度报告，接收数据进度报告操作
Synchronously read data from the MQTT server, send the specified encoded string payload to the server, and then return the data from the server, and convert it to the specified encoded string, support data transmission progress report, the server executes the progress report, and receives the data progress report