[FEATURE]: Add `getSubqueryFields` helper to goodies

### Describe what you want

The `getTableColumns` helper is really useful, but an equivalent lacks for subqueries. This can easily be implemented in userland, although it feels like the kind of helper that should come with `drizzle-orm`.

Crude implementation:

```ts
export function getSubqueryFields(
query: WithSubqueryWithSelection | SubqueryWithSelection
): (typeof query)['_']['selectedFields'] {
return query[SubqueryConfig as unknown as string].selection;
}
```