Add a configuration to choose how to encode object-only ADTs

[mbaechler]

As you can see here #1300 , ADTs are not encoded the same way in scala 2 and scala 3.

Looking at the scala 2 code, it's just lagging behind in term of feature (it doesn't detect object-only ADTs whereas scala 3 code do).

We could synchronize both versions.

But on some projects, we rely on the scala 2 encoding and we would need to be able to choose this encoding in both scala 2 and scala 3.

By the way, the rational about encoding objects as { "myObject": {} } is : if/when you add a case class to your ADT, you can still parse previous ADT values. If you choose to encode as a simple value, the backward compatibility is much more tricky.

So what about adding a configuration : alwaysEncodeEnumAsObjects?

If you think the feature is a good idea, I can try to propose a PR.

What do you think?