** Implement to convert Fantom types to / from a MongoDB representation. constmixin Converter {** Converts a Mongo object to Fantom.** ** 'mongoObj' is nullable so converters can create empty / default objects.abstract Obj? toFantom(Type fantomType, Obj? mongoObj)** Converts a Fantom object to its Mongo representation. ** ** Must return a valid BSON type (or a List or Map thereof).abstract Obj? toMongo(Obj fantomObj)}