Allow for custom PK values, instead of having a default handled by python/db.

[Auxtal]

As the title suggests, its very frustrating for me to have to always have a default PK. I would like to reference #143. I am in a very similar if not the exact same position. I am using discord IDS as my primary keys in my DB. Now that I have starting migrating to this lib it has become apparent that that is not possible (please correct me if I am wrong).

Ideally in my opinion this should be an optional setting that users can have.

I also have a question: is it possible to use postgresSQL schemas because from my understanding it isn't, that would be a useful feature as well.