(Reuters) – China’s 20th Communist Party Congress kicks off on Sunday, with Xi Jinping widely expected to clinch his third five-year stint in charge – a mandate that would secure his stature as the country’s most powerful ruler since founding leader Mao Zedong. * Little is known about who will be promoted into which key roles on the Politburo and its seven-member Standing Committee. Here are some of the leading contenders and some scenarios to look out for. * Leaders face a difficult time, with a slowing economy, questions over whether and how to exit its strict zero-COVID policy, rising tensi…