IT seems an impossible dream, if not the one that the Man of La Mancha crooned onstage. But here it goes: Over the next decade, dozens of American cooks schooled in the authentic cooking of Spain and trained in Spanish restaurants will begin to populate the United States. In due time, hundreds, then thousands, will serve up a cuisine that is not Mexican, or Caribbean, or Latin American, but one faithful to Spain. Not only will they staff a national roster of credibly Spanish restaurants, but they will go on to create new ones. Ultimately, that will ramp up American demand for the wine and products of Spain.