Returns the suppression-list picker for a campaign: suppression_lists is every suppression list in the account (each with its UUID and name), and selected_suppression_list_uuids is the subset currently applied to the campaign. Use this to render a selection UI, then persist changes with save_campaign_suppression_lists. This route accepts either GET (with campaign_uuid as a query-string parameter) or POST (with the JSON body shown here).