By David Brunnstrom and Humeyra Pamuk WASHINGTON (Reuters) -White House Indo-Pacific coordinator Kurt Campbell said on Tuesday that it was possible for China and United States to coexist in peace but the challenge was enormous and Beijing had become increasingly assertive. Campbell told the Asia Society think tank President Joe Biden will host a summit later this year with the leaders of Australia, India and Japan - the so-called "Quad" group Washington sees as a means of standing up to China. Asked when he expected a first meeting between Biden and Chinese leader Xi Jinping and whether this c...