A Randomized, Double-blinded, Multicenter, Phase Ⅲ Clinical Study of HLX22 (Recombinant Humanized Anti-HER2 Monoclonal Antibody Injection) in Combination With Trastuzumab and Chemotherapy (XELOX) Versus Trastuzumab and Chemotherapy (XELOX) With or Without Pembrolizumab for the First Line Treatment of Locally Advanced or Metastatic Gastroesophageal Junction and Gastric Cancer