Colombia’s new leftist leader is proposing steps to legalize elements of his country’s flourishing narcotics industry, signaling a potential break with a past hard-line strategy on drugs and a test of Bogotá’s ties with its most powerful ally, the United States. President Gustavo Petro, a former guerrilla whose election this summer marked an end to decades of conservative rule in Colombia, described plans that would permit small-scale farmers to legally grow coca leaf, the raw material for cocaine, and address deforestation and climate change by paying farmers not to plant the crop — or anything else — in Colombia’s rainforest.