Read a campaign ad schedule

Returns the campaign's ad schedule: which days and hours it may serve, and the bid modifier on each block. Use when: the user asks when ads run, or before changing a schedule so you can show what is there now. Do not use when: you want hour-of-day performance — that is google_get_hourly_performance, which is what tells you whether a schedule is a good idea. An empty schedule means the campaign runs all day, every day: that is Google's default and is not a problem by itself. Hours are in the account timezone, and end_hour 24 means midnight. If you are unsure which account, campaign or period the user means, ask them — do not guess ids or dates.