Ability to add additional fields without custom templates.

Hello,
https://entgo.io/docs/schema-fields#additional-struct-fields is not so handy to use and is not explicit, it would be great to have ability to create additional fields right in schema.

- [x] I have searched the [issues](https://github.com/ent/ent/issues) of this repository and believe that this is not a duplicate.

## Summary 💡
I suggest adding an annotation, with name like field.NoStorage() or field.Additional().

## Motivation 🔦
It would be way more easy to deduce for new project developers how this additional field appeared in schema they look at.