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 summit in Los Angeles whose agenda has been undermined by discord over the guest list. Seeking to counter China’s growing clout in the region, a senior administration official said President Joe Biden is offering neighbors to the south an alternative that calls for increased U.S. engagement, including more investment, strengthening supply lines and building on existing trade deals. …