Send a stop speaking command to clients

[Seraf]

Server should be able to send command to clients.
I will use the "type" field and a value "command", then in the body, I will put the command.
Plugins (and other clients) will be able to ask clients to stop speaking.
It will allow to send other command like entering a context mode, ask a question to the user and waiting for an answer without waiting for the triggers, etc ...