[FEATURE]: Single Table Inheritance

### Describe what you want

Hello,

I would really like to see support for [single table inheritance](https://en.wikipedia.org/wiki/Single_Table_Inheritance) in drizzle.

Single table inheritance is a way to save inheritance structures to a single database table. Imagine a `accommodation` class in OOP, which can be either a `hotel` or a `motel`. Both sub-classes inherit a set of properties from the parent (only `id` in this case) but each class also has additional fields (`hotelName` or `motelName`) that are unique to their specific class.

You could save it in the database like this:

| type | id | hotelName | motelName
| ------------- |:-------------:| -----:|-----:|
| hotel | 1 | Hotel-Name-1 | NULL
| motel | 2 | NULL | Motel1
| hotel | 3 | Hotel-Name-2 | NULL

Prior work in other ORMs:
[TypeORM Node.js](https://orkhan.gitbook.io/typeorm/docs/entity-inheritance#single-table-inheritance)
[CycleORM PHP](https://cycle-orm.dev/docs/advanced-single-table-inheritance/current)
[Hibernate Java](https://www.baeldung.com/hibernate-inheritance#single-table)

Possible API:

```typescript
pgTable({
id: serial("id"),
type: text("type"),
})
.addChildEntity({
type: 'hotel',
hotelName: text("hotelName"),
})
.addChildEntity({
type: 'motel',
motelName: text("motelName"),
});
```

The output type should be a discriminated union for: `type Table = Hotel | Motel` based on the value of `type`.
Not sure how well this API works out in practice (type inference, relations?).

Thanks