SSML for individual cards or attachments

[tomezpl]

Is your feature request related to a problem? Please describe.
In one of the Dialogs provided by our chatbot, we make use of carousel collections. As an accessibility consideration, we aim to always provide text-to-speech for all areas of the bot. However, it appears SSML can only be attached to a whole Message Activity, rather than to individual Attachments contained within it.

As an example, the user can ask "i'd like to book a room", and the bot would respond with a carousel of Hero Cards describing each suitable room, as well as a top-level message text for the Activity itself (e.g. "I found 5 rooms, please choose"). At that point, our only options are to either compile and play an overly long spoken message for all rooms at once, or don't provide text-to-speech at all. The former is poor UX, and the latter abandons accessibility altogether.

Describe the solution you'd like
Ideally, either Attachment or Card types should contain a ssml property. In our DirectLine-based Web client, we could then implement the speech playback as the user switches between the cards within the collection, only playing the speech relevant to the card they're focused on.

Describe alternatives you've considered
An alternative solution we are sticking with for the time being is including the SSML in the Activity's channelData, as in our case we only provide the chatbot via our own DirectLine client, so we're able to make use of that.