Add a 'scoring' argument to SplineCV

[leouieda]

Description of the desired feature:

As pointed out in https://github.com/orgs/fatiando/discussions/61 by @JamesSample, the SplineCV class never got a scoring argument when it was added to cross_val_score. This means that it's impossible to set a custom scoring function or even prepare the code for the change coming in 2.0 (see #322).

We need to add a scoring parameter to SplineCV that defaults to None. It should be passed along to the call to cross_val_score here: https://github.com/fatiando/verde/blob/main/verde/spline.py#L225

The call above that doesn't need to be changed since the client argument is going away as well soon.

A test would be needed to make sure that the scores_ attribute match a negative MSE instead of R² when the argument is passed.

Are you willing to help implement and maintain this feature? Yes but happy for someone else to do this!