Implement and test `unique-columns` feature inside `sql-connector`

[ozgrakkurt]

Currently it is implemented only on json-sql and it concatenates them into a string. This isn't ideal since it isn't tested and it isn't documented and implemented inside sql-connector.

Ideally we want to implement/test/document it inside sql-connector and use this api from json-sql.

current hacky implementation

[TheSamsa]

I was wondering why it was suddenly a &str thanks for the code.
Also would suggest removing this conversion in the smart module, and move it to the sql-connector itself. Probably put it into the insert.rs or upsert.rs modules.