Implement Rotosolve for non-equidistant frequencies in the cost function

[dwierichs]

Feature details

This is the follow-up step of #1489 , leveraging our paper on general parameter-shift rules and cost function reconstructions to apply Rotosolve not only to functions with arbitrary integral frequency spectrum, but also to non-equidistant frequencies.

Implementation

Essentially, cases have to be tracked but most of the work is done already, as the reconstruction for general frequency spectra is implemented in qml.fourier.reconstruct via #1864 and the 1D minimization does not have to change based for the generalization.
The structure of Rotosolve likely will change a bit, but also code duplication from reconstruct will be removed.

How important would you say this feature is?

2: Somewhat important. Needed this quarter.

Additional information

I'm working on a prototype.