Allow "local app bots" to post messages via an Event

[nickvergessen]

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Feature request

Which Nextcloud Version are you currently using: main

Is your feature request related to a problem? Please describe.

You can find the code on https://github.com/ahti/nextcloud-talk-incoming-webhook-bot . The spiciest bit is probably WebhookService.sendMessage (I wanted to go the event based route and that's what worked to send messages without having a messsage to respond to).

Describe the solution you'd like
A clean API to post a message

Describe alternatives you've considered
Invoking ChatManager directly