FFT extension

[tomwhite]

Implement the Fourier transform extension from the Python array API standard.

[alxmrs]

I just encountered this implementation of a distributed FFT, though it might be a useful reference: https://github.com/NVIDIA/physicsnemo/blob/00afc1f0fb599d8ae340a7e0c51d910f2c92a50b/physicsnemo/distributed/fft.py#L66