Create a LinkedIn campaign group (Campaign Manager campaign)

Creates a campaign group (a campaign in Campaign Manager) — the container every LinkedIn campaign (ad set) must sit inside, and the place a shared lifetime budget can be set across several campaigns. Use when: starting a distinct initiative (a launch, a quarter, a market) that should be budgeted and reported as one thing, or when a create tool needs a group and no sensible existing one is there. Do not create a group per campaign: that is what campaigns are for, and a forest of one-campaign groups makes the account unreadable. Run linkedin_list_campaign_groups first and reuse. Do not set a total budget the user has not named. A group budget is a hard ceiling: when it is reached every campaign inside stops, whatever their own budgets say. LinkedIn only accepts a total budget together with an end date; if the user gave no end date, ask for one. The group is created ACTIVE, because LinkedIn creates groups only as ACTIVE or DRAFT. It holds no campaigns, so nothing spends, and every campaign created inside it starts paused. Returns the group read back from LinkedIn.