Ability to serialize fields explicitly

[jonatino]

Sometimes you want to serialize a large class but only serialize 1 or 2 fields within it. It's tedious to have to @Transient dozens of fields in order to only serialize 2. It would be nice if there was a configuration option for @Serializable which only serializes fields that are explicitly tagged with an @Expose annotation (or something similar).

Gson offers this feature.

See new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create() and @Expose

Api could be something similar to

@Serializable(explicit = true)

and

@Expose for the fields we want to serialize or @Explicit.