Support sealed interface data schemas

From Slack discussion, there can be a situation when an input file can have one of few specific data schemas. It would be nice to be able to apply transformations to different schemas, basically like dynamic dispatch in OOP. With some code generation, it's possible:
```
@SealedDataSchema
sealed interface Test {
val a: Int

companion object
}

@DataSchema
interface Test2 : Test {
val b: String
}

@DataSchema
interface Test3 : Test {
val c: String
}
```

generated code:
```
sealed class Wrapper {
abstract val df: DataFrame
}

class Wrapper2 : Wrapper {
override val df: DataFrame
}

class Wrapper3 : Wrapper {
override val df: DataFrame
}

fun Test.Companion.of(df: DataFrame<*>): Wrapper? {
TODO("need to figure out this part, need to pick most suitable schema for df")
}

// might be useful to have same for DataRow
fun Test.Companion.of(df: DataRow<*>): Wrapper? {
TODO("need to figure out this part, need to pick most suitable schema for df")
}
```

use:
```
fun transform(df: DataFrame<*>) {
val typed = Test.of(df)
when (typed) {
is Wrapper -> typed.df.a
is Wrapper2 -> typed.df.b
is Wrapper3 -> typed.df.c
null -> ...
}
}
```

No repository files or tests are named. Start by reviewing the Slack discussion referenced in the issue and the sealed-schema, DataFrame, and DataRow examples to define schema selection and wrapper behavior; done would mean a decided design for supporting these transformations.

Written by the indexing model from the issue text.