Standalone Rich Business Messaging (RBM) rich card. Rich cards allow
you to respond to users with more vivid content, e.g. with media and
suggestions. You can group multiple rich cards into one using [RbmCar
ouselCard][google.cloud.dialogflow.v2beta1.Intent.Message.RbmCarouselC
ard] but carousel cards will give you less control over the card
layout.
Required if orientation is horizontal. Image preview alignment
for standalone cards with horizontal layout.