Add custom method for `plan_CUDA_FFTW`

[roflmaostc]

Similar to here, but more general: https://github.com/roflmaostc/PtyLab.jl/blob/8f79d6340630829adf2fccbe2117d460f9c33e88/src/Operators.jl#L8

[roflmaostc]

tagging @LukasHasf

[roflmaostc]

Drawback is that we need a CUDA dependency for it :/