API/DOC: Add `NumpyEADtype` to the public API

[mroeschke]

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?

[ikigai27]

can i get some additional information @mroeschke

[mroeschke]

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