Load virtual into schema struct

[jswanner]

References:

• Repo.load/2 doesn't cast virtual fields into a Schema struct #4398

[josevalim]

It looks good to me, although I am a bit worried about loading virtual fields. Do you have thoughts @greg-rychlewski? Worst case scenario we require it to be opt-in via a flag.

[greg-rychlewski]

I think it would have to be opt-in because the function is shared by embedded loading. So if it wasn't opt-in virtual embed fields would start appearing in results.

Using the schema as the type definition feels a bit off to me if wanting to load virtual fields. They are ignored in all the other Schema API functions, as far as I can see.

Maybe it would make sense to expand the reflection api so the user can create their own type map?