Interfaces should be used everywhere instead of types

[provokateurin]

Currently it is not possible to fully utilize the type hierarchy because many places just assume the type instead of it's interface:

https://github.com/nextcloud/neon/blob/42551c66386da2740bf10b21b363f183bab55bd6/packages/nextcloud/lib/src/api/spreed.openapi.dart#L16655 should be typedef Room_LastMessage = ({$BaseMessageInterface? baseMessage, BuiltList<Never>? builtListNever, $ChatMessageInterface? chatMessage}); instead.

https://github.com/nextcloud/neon/blob/42551c66386da2740bf10b21b363f183bab55bd6/packages/nextcloud/lib/src/api/spreed.openapi.dart#L16759 should be $OCSMetaInterface get meta; instead.

[Leptopoda]

I recently thought about this issue when working on cookbook and some initial tests where sadly quite breaking and iirc also having issues with the serializer.

I also thought about hiding the default implementation similar to the blocs in the neon code but this would hide the BuiltValue type which was also undesirable:

class TestInterface{
factory TestInterface() = Test;
}

class Test implements TestInterface{
}

I'll take another look in the coming days.

[Leptopoda]

Ok this is really hard to do with our current dynamite code.
I'm already working on changing this limitation but it will take some time.

Hiding the implementation and exposing it through a factory in the interface is not possible for various limitations in built_value_generator.

1. it can not generate serializers for hidden classes (due to potential naming conflicts)
2. it does not allow any constructor on an interface class (could be changed upstream but I doubt it will be accepted)

[provokateurin]

A simple but dirty workaround for this is calling toJson/fromJson to convert between classes that use the same interface.