Show `filterable` flag if attribute is `unique`

[lukashornych]

In evitaDB if attribute is unique, it is automatically filterable but the explicit flag filterable is still empty in such case. We would like to state this fact in schema viewer in evitaLab as a side note maybe, something like:

(since the attribute is unique, it can be filtered by it)

We don't want to just simply toggle filterable flag without the side note, because it could indicate that developer explicitly set the attribute as filterable.