Description
Add the ability to add clickable responses to a ntfy message. This is allowed in android notifications (called notification actions: https://developer.android.com/training/notify-user/build-notification.html#Actions). To make it consistent across all the ways to view a message I think it would also need to be added to the ntfy app's list of messages (adding clickable buttons under the message) and to web push notifications.
The action taken when a button is pushed could be a message published on a topic determined by a header set in the original notification and text indication which button. In this way the server that sent the actionable notification can just listen on a determined topic for a reply and to take action. The number of buttons and their text could be set in the same way.
Action buttons in a android notification (concept):
