[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

Start with the IoTDB built-in MQTT service and the V0.12.x MQTT programming guide linked in the issue, focusing on how MQTT JSON messages are handled when no timestamp is present. Determine the expected database-insert timestamp behavior and verify it with coverage for messages both with and without an explicit timestamp.

Written by the indexing model from the issue text.