Carousel Rich Business Messaging (RBM) rich card. Rich cards allow
you to respond to users with more vivid content, e.g. with media and
suggestions. If you want to show a single card with more control over
the layout, please use [RbmStandaloneCard][google.cloud.dialogflow.v2b
eta1.Intent.Message.RbmStandaloneCard] instead.
Required. The cards in the carousel. A carousel must have at
least 2 cards and at most 10.