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 (data-apis/array-api-extra#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.

[asmeurer]

Was there a reason for this or was it just an accidental omission? Mean is well-defined for complex numbers (https://en.wikipedia.org/wiki/Complex_random_variable), and should be easy to implement since you either just apply the real kernel formula directly to complex data types, or take the means of the real and imaginary parts separately.

[kgryte]

I believe an omission and thus far no R&D whether supported across all array libraries.

[kgryte]

I've opened #850.