[FEATURE]: NULLS (NOT) DISTINCT for Postgres Unique Indexes

### 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

A very useful new-ish feature (afaict, Postgres 15) is NULLS NOT DISTINCT in Postgres unique indexes.
https://www.postgresql.org/docs/current/indexes-unique.html

That is already supported for `unique` but not for `uniqueIndex` in `drizzle-orm`.
https://orm.drizzle.team/docs/indexes-constraints#unique

The API can definitely be same as `unique`
```ts
export const userNulls = pgTable('table', {
courseId: uuid().references(() => course.id),
lessonId: uuid().references(() => lesson.id),
}, (t) => [{
unq: uniqueIndex().on(t.courseId, t.lessonId).nullsNotDistinct()
}]);
```