Add a drift detector base class

[ascillitoe]

We should explore adding a base drift detector class. This could help with a number of issues:

1. The preprocess methods are heavily duplicated across drift detectors (in cd.base):

        if self.preprocess_fn is not None:
            x = self.preprocess_fn(x)
            if not self.preprocess_at_init and not self.x_ref_preprocessed:
                x_ref = self.preprocess_fn(self.x_ref)
            else:
                x_ref = self.x_ref
            return x_ref, x  # type: ignore[return-value]
        else:
            return self.x_ref, x  # type: ignore[return-value]
   

2. The preprocess logic in detector __init__ methods is heavily duplicated:

        # x_ref preprocessing
        self.preprocess_at_init = preprocess_at_init
        self.x_ref_preprocessed = x_ref_preprocessed
        if preprocess_fn is not None and not isinstance(preprocess_fn, Callable):  # type: ignore[arg-type]
            raise ValueError("`preprocess_fn` is not a valid Callable.")
        if self.preprocess_at_init and not self.x_ref_preprocessed and preprocess_fn is not None:
            self.x_ref = preprocess_fn(x_ref)
        else:
            self.x_ref = x_ref
   

3. Some args/kwargs and methods are shared by all drift detectors i.e. input_shape and data_type. A base class would ensure these are included in new drift detectors (similar to an abstract method).

We should also check the super().__init__() calls in classes like MMDDrift. Some may not be neccesary.