Generated code does not support round-tripping of unknown enum/union types

[nmiyake]

The Conjure specification stipulates that clients should be able to round-trip unknown variants of enums and unions: https://github.com/palantir/conjure/blob/master/docs/spec/wire.md#44-round-trip-of-unknown-variants

This means that, even if a union unmarshals into an "unknown" type, it should be able to re-marshal the unknown union into its original form.

Currently, the generated code does not do this -- although it will unmarshal unknown variants, the serializer code returns an error when an unmarshaled unknown is re-marshaled.

In order to fix this, the generated unions will likely need to store the raw JSON and re-write that when it is marshaled. Not sure if there's a straightforward way to properly support this for YAML, but at the very least we should match the required JSON behavior.

[nmiyake]

@bmoylan for SA since he's actively doing work in the area of Conjure serialization right now