Method that returns the frequency at a specific index on a Spectrogram

### Describe the feature

Having a method that converts between indices and frequencies for `GenericSpectrogram` could be really useful for ML or other post-processing operations.

### Proposed solution

I am not sure if it will work for all providers, but if `.freq_axis()` is available, the information should already be there. Something like `.freq_at_index(idx)`