Campaigns
Get campaign by ID
campaigns.retrieve(id) -> CampaignRetrieveResponse { campaign }
GET/campaigns/{id}
Cancel a campaign
campaigns.cancel(id) -> CampaignCancelResponse { success }
POST/campaigns/{id}/cancel
Export campaign rows
campaigns.export_rows(id) -> CampaignExportRowsResponse { csv, filename }
GET/campaigns/{id}/export