ENH: Move identity to the ArrayMethod to allow customization

This also fixes/changes that the identity value is defined by the
reduce dtype and not by the result dtype.  That does not make a
different in any sane use-case, but there is a theoretical change:

    arr = np.array([])
    out = np.array(None)  # object array
    np.add.reduce(arr, out=out, dtype=np.float64)

Where the output result was previously an integer 0, due to the
output being of `object` dtype, but now it is the correct float
due to the _operation_ being done in `float64`, so that the output
should be `np.zeros((), dtype=np.float64).astype(object)`.

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