ww_multi_scale() should support multi-class AUC, PR AUC, etc

[mikemahoney218]

Because ww_multi_scale() only takes one estimate column, and passes ... to sf::st_make_grid(), there's currently no way to pass multiple columns of probability estimates to yardstick functions. This means we can't do multi-class class probability functions at the moment.

It would be nice for at least the sf method to support this. Not sure what the interface would look like, though, since dots are already in use. Maybe a probabilities argument that accepts a list?