List the contacts that opened a campaign, with how many times each of them opened it. This is the v3 equivalent of the v1 campaign_report_open_list action.

One page of rows is returned and no row count, so page until a short page comes back: the response carries no meta.total.

Requires the Campaigns feature and access to the campaign; a campaign the user cannot see is reported as 404 so the endpoint does not disclose which campaign ids exist.