Brian Steinberg Senior TV EditorTV usually belongs to the celebrities who play the nation’s favorite characters — people such as Ellen Pompeo, the star of ABC’s “Grey’s Anatomy,” or Milo Ventimiglia from NBC’s “This Is Us.” During the industry’s upfront market, however, many eyes move instead to the network ad-sales chiefs.These execs will be out in full force over the next several weeks, eager to keep the tens of billions of advertising dollars flowing into their companies’ corporate coffers.