"createFromJson" for `@JsonSerializable` to turn on/off deserialization code generation

Hi,

I have a class that only requires serialization to JSON but not deserialization.

For specific fields there is `includeFromJson` property for `JsonKey`.

Can we get the same for the whole class?

```dart
@JsonSerializable(createFromJson: false)
class MyClassThatOnlyNeedsSerialization {
// ...
}
```

Thanks! :)