API: public way to get the "best values" of a Series?
For a use case in pyarrow, I need to get the underlying values of a Series: an ExtensionArray if it is back with one, or otherwise the numpy array.

Is there public API to get this? We have `Series.array`, but this always returns an ExtensionArray, and we have `Series.values`, but this return a numpy array for eg periods (for historical reasons).

For Index, we have the private `Index._values` described in the docstring as the "best array representation". And I think `Series._values` is somewhat similar.

But the question is: do we want a public way to get to this? I am personally not sure we should, as there are still dubious cases (like datetime64/timedelta64, for this one, `Index._values` and `Series._values` is actually different ...). 

But if we don't add it, do we have a recommended way for external projects to do this? 
(basically it is something like the `extract_array(..., extract_numpy=True)` ?) 

cc @TomAugspurger @jbrockmendel @jreback 

