Add search themes to an asset group

Adds search themes to a Performance Max asset group. A search theme is a phrase telling Google what customers look for; it is a hint, not a keyword, and Google may serve on plenty of queries you never listed. Use when: the campaign is new and has nothing to learn from, or the user knows terms the ads should cover. Do not use when: the user expects keyword-level control. Performance Max has none, and search themes do not add any. Say so rather than implying otherwise. An asset group holds at most 25 themes; the tool refuses if the addition would go over, and themes that are already there are skipped rather than duplicated. If you are unsure which account, campaign or period the user means, ask them — do not guess ids or dates.