Add query.whereNative

[Paxa]

This is for custom code in SQL conditions. For example

query.whereNative("ST_Contains(city.geom, superhero.geom)");
// or
query.nativeWhere("exists (select 1 from t1 where .. limit 1)");

Probably there can be a better name for method... If you think it's good I will write implementation for other drivers and add some tests