RFC: add support for complex input to `mean`

[lucascolley]

x-ref https://github.com/data-apis/array-api-extra/pull/10/files#r1776010210

While implementing a simple cov in array-api-extra (https://github.com/data-apis/array-api-extra/pull/10) I found that xp.mean does not support complex input. @kgryte suggested opening an issue over here as the arithmetic mean is well-defined for complex numbers.