Support encoding Pickler discriminator with oneOfWrapped

Similarly to Schema.oneOfWrapped, we want Pickler.oneOfWrapped for sealed hierarchies. This way, we'll get JSON encoding like

{"userStatus": { "Closed": { "description": "account deleted" } } }

instead of current

{"userStatus": { "$type": "Closed", "description" : "account deleted" } } }

for

sealed trait UserStatus
case class Closed(description: String) extends UserStatus

case class User(userStatus: UserStatus)

Challenges:

1. Calling Schema.oneOfWrapped[T] causes underlying macro to summon schemas for all child subtypes of T. We would prefer passing these child schemas as a parameter, because they are available from child picklers.
2. In Writes trait we should probably override write0 when creating a new coproduct writer, and detect there that we're dealing with a special type of SubtypeDiscriminator. Then, the logic of write0 should be low-level uPickle code that writes the wrapper object.

Start by tracing Schema.oneOfWrapped and the Pickler.oneOfWrapped entry point, then inspect the Writes trait's coproduct-writer handling and SubtypeDiscriminator logic. Compare the resulting JSON with the requested wrapped form for the UserStatus example while preserving existing discriminator encoding.