Attach an existing brand list to one or more campaigns — returns a PREVIEW.
Creates a CampaignCriterion.brand_list per campaign. This is NOT the
CampaignSharedSet linkage used for negative keyword lists — brand lists are
criteria, and `negative` decides the role. The shared_set_id comes from
get_brand_lists; get_brand_list_campaigns lists existing attachments.
Returns a preview with a plan_id; nothing changes until the returned
plan_id is applied with confirm_and_apply.
Google Ads API: https://developers.google.com/google-ads/api/docs/start
ConnectorAPI key