Add structured snippet extensions

Creates a structured snippet asset — a fixed header such as "Types" or "Brands" followed by 3–10 values — and links it to the account, a campaign or an ad group. Use when: the user has a list that genuinely fits one of Google's headers. Do not use when: no header fits. Google only accepts its own header list (Amenities, Brands, Courses, Degree programs, Destinations, Featured hotels, Insurance coverage, Models, Neighborhoods, Service catalog, Shows, Styles, Types); inventing one fails. Callouts are the right tool for free-form selling points. The header must be Google's exact translation for the language of the ads. Pass header in English and, when the ads are not in English, language as Google's locale code (for example "de", "fr", "ja", "pt-BR", "zh-TW"): the tool sends Google's translated header, and refuses a header Google has not translated for that locale. When language is left out the header is sent in English and the preview warns if the campaign targets other languages. If you do not know the language of the ads, ask. Each value is 25 characters or fewer (12 in Chinese, Japanese or Korean) and at least three values are required. An identical header+values snippet already linked at the same level is skipped. If you are unsure which account, campaign or period the user means, ask them — do not guess ids or dates.