[Apache-IoTDB-MQTT] request to add timestamp automatically, if not provided by MQTT

We are having a lot of very small embedded boards (Arduino) without a RTC function (Real-Time-Clock). If we want to insert the data over MQTT, with the IoTDB-built-in-MQTT-service, the insertion only works, if we provide the timestamp inside the MQTT-JSON-format, like on your example: https://iotdb.apache.org/UserGuide/V0.12.x/Communication-Service-Protocol/Programming-MQTT.html

It would be nice, if the timestamp can be added automatically by IoTDB on database insert, if no timestamp is provided inside the MQTT-JSON structure.

IoTDB Version 0.12.1

从 IoTDB 内置的 MQTT 服务以及 issue 中链接的 V0.12.x MQTT 编程指南入手，重点关注不存在时间戳时 MQTT JSON 消息的处理方式。确定插入数据库时预期的时间戳行为，并通过覆盖有显式时间戳和没有显式时间戳的消息来验证这一点。

由索引模型根据 Issue 内容生成。