ENH: use EA.argmin/argmax in EA._reduce / Series.argmin/argmax

[jorisvandenbossche]

Follow-up on #27801: that PR added ExtensionArray.argmin()/argmax() methods, but this is not yet used in the equivalent Series methods (through _reduce).

When adding this, we will need to decide on the following issues as well:

• API/BUG: Series.argmin/max with all-NaN data returns -1 ? API/BUG: Series.argmin/max with all-NaN data returns -1 ? #33941
• API: argmin/argmax behaviour for nullable dtypes with skipna=False API: argmin/argmax behaviour for nullable dtypes with skipna=False #33942

[TomAugspurger]

I have a branch started for this at https://github.com/TomAugspurger/pandas/tree/32749-seriesidxminmax. Ran into the skipna returning -1 issue.

[jbrockmendel]

Series.argmin is defined in base.IndexOpsMixin and does call delegate.argmin(). Is the idea here that going through _reduce is important?