Symmetry Breaking and Cooperative Spin Crossover in a Hofmann-Type Coordination Polymer Based on Negatively Charged {Fe<sup>II</sup>(μ<sub>2</sub>-[M<sup>II</sup>(CN)<sub>4</sub>])<sub>2</sub>}<sub>n</sub><sup>2n–</sup> Layers (M<sup>II</sup> = Pd, Pt)