Capabilities the device surface supports at the time of the request.

UNSPECIFIED Unspecified surface capability.
SPEECH Device can speak to the user via text-to-speech or SSML.
RICH_RESPONSE Device can display rich responses like cards, lists and tables.
LONG_FORM_AUDIO Device can play long form audio media like music and podcasts.
INTERACTIVE_CANVAS Device can display a interactive canvas response.
HOME_STORAGE Device can support saving and fetching home storage.