By David Brunnstrom, Alexandra Alper and Michael Martina WASHINGTON (Reuters) -The United States began a first-of-its-kind summit with Pacific island leaders on Wednesday, saying they had agreed a partnership for the future and holding out the prospect of “big dollar” help to a region where it hopes to stem China’s expanding influence. Leaders from 12 Pacific island states were expected to take part in a two-day summit in Washington, with two more sending representatives, and Australia and New Zealand attending as observers. White House Indo-Pacific coordinator Kurt Campbell said last week the…