Implement type-safe custom fields

[Akii]

Basic idea:

@Generated
public sealed interface OrderCustomFieldsType : CustomFields

@Generated
public sealed interface CategoryCustomFieldsType : CustomFields

@Generated
public class TypeACustomFields @JsonCreator constructor(
  @JsonProperty("custom")
  custom: CustomFieldsImpl,
  @JsonProperty("typedFields")
  public val typedFields: Fields
) : CustomFields by custom, OrderCustomFieldsType, CategoryCustomFieldsType {
  @Generated
  public data class Fields @JsonCreator constructor(
    @JsonProperty("aBoolean")
    public val aBoolean: Boolean?,
    @JsonProperty("aReference")
    public val aReference: ProductReference?
  )
}

@Generated
public class TypeBCustomFields @JsonCreator constructor(
  @JsonProperty("custom")
  custom: CustomFieldsImpl,
  @JsonProperty("typedFields")
  public val typedFields: Fields
) : CustomFields by custom, CategoryCustomFieldsType {
  @Generated
  public data class Fields @JsonCreator constructor(
    @JsonProperty("anotherBoolean")
    public val anotherBoolean: Boolean?
  )
}

val foo: OrderCustomFieldsType = null as OrderCustomFieldsType

// check is exhaustive
when (foo) {
    is TypeACustomFields -> foo.typedFields.aBoolean
}