[FEATURE]: Improved types for conditions

### Feature hasn't been suggested before.

- [x] I have verified this feature I'm about to request hasn't been suggested before.

### Describe the enhancement you want to request

Conditions such as `gte` or `eq` are SQL expressions that resolve to a boolean value. However, they are typed as `SQL` (by omitting the generic) rather than `SQL`.

Having them properly typed would improve their ergonomics, specially when building helpers for more advanced expressions, such as the [`CASE` expression](https://www.postgresql.org/docs/current/functions-conditional.html#FUNCTIONS-CASE).