customizable codegen

[lolopinto]

• add more fields to generated ent class
• changes to builder?
• changes to action?
• add more graphql properties e.g. https://github.com/lolopinto/ent/pull/1506 should have been possible without ent change. developer could just have modified something in codegen to add the property to the given class
• subscription in GraphQLSchema could maybe be done this way https://github.com/lolopinto/ent/issues/1544