The two parking spaces closest to the South End’s Wholy Grain cafe are just like thousands of others in Boston: patches of public asphalt hard against the curb, reserved for private vehicles. By this time next year, they could be a park. City planners are refining a pilot program, modeled after San Francisco and New York, to turn spaces here and in three other neighborhoods into petite, three-season patios, with benches and planters atop platforms built flush with the sidewalk.