Allow subclasses for @AutoGen

I suggest adding a new @AutoGen(passthrough = true) option specifically to enable this kind of thing:

@SerialEntry
@AutoGen(category = CHEATS, passthrough = true)
 // this means that the field won't be converted to an option
 // instead the fields of this object will 
public Cheats cheats = new Cheats();

public static final class Cheats {
    @SerialEntry
    @TickBox
    @AutoGen // All of the fields generated under the category CHEATS
    // The category field has to be optional and have a usage contract instead 
    public boolean expendWandChargeInSurvival = true;
}

No source file or test is named. Start by tracing how the @AutoGen annotation handles category values and generated fields, then compare that behavior with the nested Cheats example. Done means a passthrough option can generate nested fields under the parent category without converting the parent field itself into an option, with the category field remaining optional.