Optionally allow encoding PhantomData<T> types

[SkymanOne]

paritytech/substrate#14750 introduces the persistence of some configured types in the metadata. It uses PhantomData<T> to express these types as they don't need to get instantiated.

However, due to #111 these types are not reflected in the metadata.

Is it possible to add an optional flag to persist these fields in the metadata?

[athei]

cc @ascjones

[ascjones]

I suggested using a wrapper type, let me know if that works

[athei]

Thanks. That works.