By Humeyra Pamuk, Matt Spetalnick and Dave Graham LOS ANGELES (Reuters) -The Biden administration unveiled a new proposed U.S. economic partnership with Latin America on Wednesday as regional leaders gathered for a U.S-hosted summit whose agenda has been undermined by discord over the guest list. Seeking to counter China’s growing clout, a senior administration official said President Joe Biden is offering America’s neighbors to the south an alternative that calls for increased U.S. engagement, including stepped-up investment, strengthening supply lines and building on existing trade deals. Ho…