Why is `unique` not stored as property in `FieldInfo`?

#### Question about pandera

It is technically not a bug since nothing breaks, so I'll ask it as a question. Is there any particular reason why is `uniqueness` not returned from `properties()` in `FieldInfo` (code [here](https://github.com/unionai-oss/pandera/blob/17c558f392e2c6f3c176cd0375a52371ae697d84/pandera/api/pandas/model_components.py#L78))? 

