Add callout extensions

Creates callout assets — short, non-clickable phrases such as "Free shipping" or "Cancel anytime" — and links them to the account, a campaign or an ad group. Use when: the user has given the phrases, or approved ones you proposed. Do not use when: the phrase is a claim the business cannot back up, contains a phone number or a URL, or repeats a headline — Google disapproves those. Each callout is 25 characters or fewer. Google shows at least two at a time, so four or more is the useful minimum. Identical callouts already linked at the same level are skipped. If you are unsure which account, campaign or period the user means, ask them — do not guess ids or dates.