ENH: fft: switch from `pocketfft` to `ducc0.fft`

Proposed new feature or change:

Scipy switched its built-in FFT library from pocketfft to its successor ducc.fft in https://github.com/scipy/scipy/pull/23436

Previous discussion in #17839. A lot of the obstacles no longer exist:

• Numpy switched to C++ pocketfft implementation, which makes it easier move to C++ ducc.fft
• The ducc.fft files were changed to be dual-licensed under BSD-3-Clause OR GPL-2.0-or-later which should be compatible with Numpy
• If we switch to ducc.fft we can also move multi-D FFT to ducc.fft instead of managing by Numpy
• Numpy doesn't have a vendoring script like SciPy. A submodule will import too many files from the ducc repo. Can we check in the vendored files manually?
• Can Scipy depends on Numpy FFT and get rid of it's own ducc.fft dependency?

I have a working local prototype, want to know how well this will be received before I proceed with a PR.

cc @mreineck