关于数据质量戳的扩展问题

目前元数据结构为值和时间戳，不少工业装备还会送上来质量戳，目前无法存储到iot中，只能再重新添加一个点，用起来就不太方便了。建议：
1）增加一个扩展字段，由用户自己定义存储的信息（一般是质量戳和报警信息）。

The issue does not name any files, tests, or entry points. Start by tracing how IoTDB stores metadata values and timestamps, then assess the design needed for a user-defined field carrying quality or alarm information. Done means the extension can be stored and retrieved without requiring a separate point, with coverage for the proposed behavior.

Written by the indexing model from the issue text.