Add support for listing unique constraints

[Niikelion]

Hello there! I find working with unique constraints across multiple columns in knex pretty cumbersome due to inability to delete all of them them/check if they exist before redefining them. This leads to the need to remember what constraints have been defined before. This is why I propose adding uniqueConstraints method that will list all unique constraints. I'm more than willing to implement this feature for some dialects(oracledb is not working on my machine).

[Niikelion]

Figured out how to make oracledb work. Now I probably can implement it for every dialect.

[Niikelion]

I managed to implement this method for every dialect. If you decide to not include this feature in knex-schema-inspector full implementation will still be on my fork.