Double-blind, randomized, placebo-controlled pilot clinical trial to evaluate the safety, tolerability and efficacy of two doses of a conditioned medium from a co-culture of M2 macrophages and fat-derived mesenchymal cells (PRS CK STORM) in the modulation of the cytokine storm for the treatment of acute respiratory distress syndrome (ARDS) caused by SARS-Cov-2, influenza A, influenza B and respiratory syncytial virus (RSV)