find_composite_key_candidates should allow users to return all possible candidate keys

[MrPowers]

find_composite_key_candidates just returns one composite key candidate now. That's good for performance.

We should also let the user figure out all possible candidate keys as well. That way they can select if they want to.

[robertkossendey]

@MrPowers I think @souvik-databricks is already on that -> #31