Support entity inheritance via "extension" tables

Allow to implement inheritance in something like:
```
object BaseTable : IdTable()
object ExtendTable : IdTable() {
val parent = reference(BaseTable)
}

class ExtendEntity : Entity {
val field1 by BaseTable.field1
val field2 by ExtendTable .field2
val dependsOnTable = BaseTable innerJoin ExtendTable
...
}

```