fix: duplicate code for extra class ?

[Cervangirard]

Looks like inside create_backend, the code to add class is not anymore needed because of super class ?
Here the code if (!is.null(backend$extra_class)) {

now, the connector R6 also give the opportunity to add the extra class, see here :

connector/R/connector.R

Line 51 in 7c2c7cc

class(self) <- c(extra_class, class(self))

So, I see pros and cons for both, may be we can keep them.

If we keep only the one inside connector, it means all connectors object will have to inherit from connector. If you forget, then extra class doesn't work. in the other hand, keep it inside create_backend will always allow users to add extra_class inside the yaml file.