enforce TypeConverter implementation test rather than inheritence in floor generator

[ramsestom]

Curently the floor generator enforce the TypeConverter instances to inherit the TypeConverter abstract class. It would make more sense to only requiere TypeConverter instances to implement the TypeConverter interface (like this is done by JsonConverter in the json_annotation library), allowing to have a single converter class that can implement multiple different converters (like floor TypeConverter, json_annotation JsonConverter, ...) if a type needs to be converted by various libraries. It would also allows to define const constructors for TypeConverter instances (which currently isn't possible because the TypeConverter abstract class do not have a const constructor).

[dkaera]

I think we may see a similar approach with converters in Retrofit, it will take some time to implement adapters for known serializers/deserializers, but overall it is a good idea.