Pandas API/DOC: Add NumpyEADtype to the public API

This is currently accessible via

In [5]: pd.array([1], dtype="int16").dtype
Out[5]: NumpyEADtype('int16')

I think this should go under pandas.api.extensions?

Comment From: ikigai27

can i get some additional information @mroeschke

Comment From: mroeschke

This issue probably needs feedback from other core devs before proceeding.