Create a campaign for a selected group

Creates a blank draft and freezes distinct contacts from a complete contact selection or email-activity drilldown. Requires campaigns:write and subscribers:read, plus analytics:read for activity. Maximum 100,000 contacts and 8 MiB stored audience JSON. No sending domain or send permission is required for drafting. Membership is saved now; active status and email eligibility are checked again at send time. Deleted historical analytics contacts are omitted. Read, replace or clear targetLists using the existing campaign endpoints.