A Randomized, Double-Blind, International Multicenter, Phase III Study to Evaluate the Anti-Tumor Efficacy and Safety of HLX10 (Recombinant Humanized Anti-PD-1 Monoclonal Antibody Injection) or Placebo in Combination with Chemotherapy (Carboplatin/Cisplatin -Etoposide) and Concurrent Radiotherapy in Patients with Limited-Stage Small Cell Lung Cancer (LS-SCLC)