Attach a message to a split campaign as an additional variant.

A copy of the given message is attached, so the response carries a message id of its own rather than the id that was sent. Because copying exposes the source message's content, the campaign the source message belongs to is authorized too.

Only split (A/B) campaigns hold more than one message. Requires the Campaigns feature and access to the campaign; a campaign that is past the point where its messages may change is a 409.