Copy a template as a new variant

Copies the template you specify into a new variant on a campaign experiment that's in <code>draft</code> or <code>ready</code> status. Each successful request adds another variant, so retrying a successful request creates a duplicate. This endpoint works only with experiments that test message content: the subject line, preheader text, from name and sender, email body, or all content. It doesn't work with send time or Send Time Decisioning experiments. This endpoint doesn't support journey experiments.