Apple’s iTunes made its mark as the place to buy music downloads, but it left the fast-growing streaming business to rivals such as Spotify, Pandora, Rhapsody and others.Now it’s playing catch-up. On Monday, Apple formally launched its bid to become a one-stop shop of digital music offering on-demand streaming, online radio and a feature where artists can share new tracks and video.Music industry veteran Jimmy Iovine, Apple’s point man on the project, called it “one complete thought around music” that stands in contrast to the “fragmented mess” of streaming and download options in the market. He told the audience at Apple’s developer conference in San Francisco that the service would be revolutionary.The big question is whether consumers will pay.