Add Regularized QuadraticModels

[MaxenceGollier]

In RegularizedOptimization.jl, in particular in the R2N algorithm, we need to model at each iteration

$$\varphi: s \mapsto f + g^T s + \frac{1}{2} s^T(H + \sigma I)s,$$

I believe we should use this package as a dependency to do so.

I started a draft but some questions remain on how to implement this.