Selectable replies to messages sent by bots [canned responses]

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Facebook's Messenger bots have a [Quick Replies](https://developers.facebook.com/docs/messenger-platform/send-api-reference/quick-replies) feature. While this maybe doesn't *quite* make sense for Matrix (since 1-1 chats aren't really a thing) I think it might be useful to allow bots to provide predefined response buttons, such that clicking/tapping them causes your client to say something in the room.

In the case of Riot/Matrix, it might make sense for these buttons to be attached to the message rather than replacing the keyboard as FB's bots do.

As a more concrete example, I have a chatbot that I use with a few friends that provides a "vote" command. You give it a question and a list of choices, and it opens a vote for users in the room:

```
#vote start "What kind of pizza should we get?" "Hawaiian" "Cheese" "Pepperoni" "Taco"
Respond with "vote CHOICE" to participate.
All: A vote about "What kind of pizza should we get?" has been called! --- jon --- Sibyldawson, schwabyl
#vote Taco
Count --- taco: 1, cheese: 0, pepperoni: 0, hawaiian: 0
#vote stop
Count --- taco: 1, cheese: 0, pepperoni: 0, hawaiian: 0
All: The vote about "What kind of pizza should we get?" has ended! --- jon --- Sibyldawson, schwabyl
```

It'd be nice if our bot could provide these choices as buttons, at which point clicking one would send the appropriate message to the room.

The issue names no files, tests, or entry points. Start by determining where Matrix message and client interaction behavior are specified; done would require an agreed protocol design for bot-provided choices and the resulting room message.

Written by the indexing model from the issue text.