By Hyonhee Shin SEOUL (Reuters) – The recent attendance by four Asia Pacific leaders at the NATO summit reflects a “consequential shift” in the U.S. transatlantic security partnership which Washington seeks to expand to better counter China, a senior U.S. diplomat told Reuters. In an interview in Seoul, Derek Chollet, counselor of the U.S. Department of State who serves as a senior policy advisor to Secretary Antony Blinken, said he sees “great potential” for cooperation between South Korea and NATO, built on previous exchanges including global efforts to help Ukraine and European countries jo…