Add a creative (ad) to a LinkedIn campaign (ad set)

Adds one more creative (an ad in Campaign Manager) to an existing campaign (ad set) — either from a new image URL (uploaded and posted as a dark post) or from a post URN that already exists. LinkedIn refuses a paused creative before its review, so inside a campaign that is not live it is created ACTIVE (reviewed now, held back by the campaign) and inside a campaign that is already ACTIVE it is created DRAFT. Use when: adding a variant to test against the current ad, or finishing a campaign whose create chain failed partway. This is the tool that repairs a campaign with no ad in it; do not re-run the create tool for that. Do not add a fifth variant to a campaign: LinkedIn's delivery optimisation splits budget across creatives and beyond three or four none of them gets enough impressions to prove anything. Do not add a creative whose image has not been checked — a failed upload here wastes a task and leaves nothing useful behind. Do not use it on a LEAD_GENERATION campaign: every ad there needs a Lead Gen Form this server cannot attach, so it is refused; add those ads in Campaign Manager. Returns the creative read back with its review status. A DRAFT creative serves nothing until the user approves linkedin_resume_creative; an ACTIVE one serves once its campaign is resumed and LinkedIn approves it.