ggml : add FFT operator

It would be nice to have a simple implementation of FFT. The main goal is to be able to keep the data inside a ggml graph in certain scenarios (such as in Whisper and various TTS models), even if the performance is not as good as dedicated libraries (such as FFTW and cuFFT). I think we can start with a basic Cooley-Tukey implementation to keep things simple. Batched 1D FFT should be supported, as well as common variations such as inverse FFT and real-valued FFT.