Google Actions for Assistant (#9632)

* http: Add headers key to json[_message]

* Add google_assistant component

This component provides API endpoints for the Actions on Google Smart
Home API to interact with Google Assistant.

* google_assistant: Re-add fan support

* google_assistant: Fix Scene handling

- The way I originally wrote the MAPPING_COMPONENT and the way it's actual
  used changed so the comment was updated to match that.
- Use const's in more places
- Handle the ActivateScene command correctly

* google_assistant: Fix flakey compare test

Was failing on 3.4.2 and 3.5, this is more correct anyway.

* google_assistant: Use volume attr for media_player

homeassistant/components/google_assistant/const.py

@@ -0,0 +1,37 @@
+"""Constants for Google Assistant."""
+DOMAIN = 'google_assistant'
+
+GOOGLE_ASSISTANT_API_ENDPOINT = '/api/google_assistant'
+
+ATTR_GOOGLE_ASSISTANT = 'google_assistant'
+ATTR_GOOGLE_ASSISTANT_NAME = 'google_assistant_name'
+
+CONF_EXPOSE_BY_DEFAULT = 'expose_by_default'
+CONF_EXPOSED_DOMAINS = 'exposed_domains'
+CONF_PROJECT_ID = 'project_id'
+CONF_ACCESS_TOKEN = 'access_token'
+CONF_CLIENT_ID = 'client_id'
+CONF_ALIASES = 'aliases'
+
+DEFAULT_EXPOSE_BY_DEFAULT = True
+DEFAULT_EXPOSED_DOMAINS = [
+    'switch', 'light', 'group', 'media_player', 'fan', 'cover'
+]
+
+PREFIX_TRAITS = 'action.devices.traits.'
+TRAIT_ONOFF = PREFIX_TRAITS + 'OnOff'
+TRAIT_BRIGHTNESS = PREFIX_TRAITS + 'Brightness'
+TRAIT_RGB_COLOR = PREFIX_TRAITS + 'ColorSpectrum'
+TRAIT_COLOR_TEMP = PREFIX_TRAITS + 'ColorTemperature'
+TRAIT_SCENE = PREFIX_TRAITS + 'Scene'
+
+PREFIX_COMMANDS = 'action.devices.commands.'
+COMMAND_ONOFF = PREFIX_COMMANDS + 'OnOff'
+COMMAND_BRIGHTNESS = PREFIX_COMMANDS + 'BrightnessAbsolute'
+COMMAND_COLOR = PREFIX_COMMANDS + 'ColorAbsolute'
+COMMAND_ACTIVATESCENE = PREFIX_COMMANDS + 'ActivateScene'
+
+PREFIX_TYPES = 'action.devices.types.'
+TYPE_LIGHT = PREFIX_TYPES + 'LIGHT'
+TYPE_SWITCH = PREFIX_TYPES + 'SWITCH'
+TYPE_SCENE = PREFIX_TYPES + 'SCENE'