Add a way to register function in a Manager

[StephaneCapponi]

It would be interesting that the manager does not register only class but also function. That way we can look for function in specific case instead of having to create a class in a single method.

The way I would do it is to implement a class decorator in the Manager. Something like:

@classmethod
def register_func(cls, func: callable):
    ...

So we can call it with any Manager on a function like:

@Manager.register_func
def myfunc(...):
    pass

The decorator must take into account in which config we are and save it in the registry.