ENH: use EA.argmin/argmax in EA._reduce / Series.argmin/argmax
Follow-up on https://github.com/pandas-dev/pandas/pull/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 ? #33941
* API: argmin/argmax behaviour for nullable dtypes with skipna=False #33942