WIP: Add sealed unions to JAVA

[artfuldev]

In this PR we allow generating sealed interfaces with permits for discriminated unions, as described in JEP-409 and made available in Java 17+

ADL supports unions, for example:

/// simple
union Authorisation {
  Void public;
  String forRole;
};

/// generic with 1 type arg
union Option<A> {
  Void none;
  A some;
};

/// generic with 2 type args
union Either<L, R> {
  L left;
  R right;
};

We simplify the output to:

public sealed interface Authorisation permits Authorisation.ForRole, Authorisation.Public {
  public record Public() implements Authorisation {}
  public record ForRole(String val) implements Authorisation {}
}

public sealed interface Option<A> permits Option.None, Option.Some {
  public record None<A>() implements Option<A> {}
  public record Some<A>(A val) implements Option<A> {}
}

public sealed interface Either<L, R> permits Either.Left, Either.Right {
  public record Left<L, R>(L val) implements Either<L, R> {}
  public record Right<L, R>(R val) implements Either<L, R> {}
}

if a specific option --sealed-unions is passed to adlc java

This means we can use pattern matching in switch expressions, and also since we use records, we get equals and hash code implementations for free.

public static String x(Authorisation a) {
  return switch (a) {
    case Public() -> "anyone";
    case ForRole(String role) -> role;
  };
}

public static <L, R> String x(Either<L, R> either) {
  return switch (either) {
    case Left(L left) -> left.toString();
    case Right(R right) -> right.toString();
  };
}

public static <A> String x(Option<A> x) {
  return switch (x) {
    case None() -> "";
    case Some(String val) -> val;
    case Some(A val) -> val.toString();
  };
}

Planned work

• Add sealed union option
• Generate sealed union code
• Support generics
• Generate backwards compatible JSON binding
• Handle parcelable
• Generate backwards compatible factory
• Fix Import::ScopedName