Pandas REF: avoid passing self as arg from ExtensionArray methods

The following functions are called from ExtensionArray methods with self as the first argument:

[ ] nargminmax
[ ] format_object_summary
[ ] rank
[ ] mode

This implicitly means that these functions are depending on EA implementation details (or in some cases just casting to object), which is a code smell. These should be refactored to use e.g. _values_for_argsort, _values_for_factorize, ...

Update Also in

[ ] NDArrayBackedExtensioNArray.argmin
[ ] NDArrayBackedExtensioNArray.argmax

Comment From: brunocoutinhoo

take

Comment From: jorisvandenbossche

Specifically for nargminmax (didn't check the others), I think that's actually on purpose? The docstring of nanargminmax says that is an implementation for extension arrays, and so that is fine to use from the EA class?