Patch Notifications with configurable JSON or Mattermost

Hi,

teams and Slack Webhook Integration is wonderfull, but for organisations without public Cloud access there are other tools like mattermost which can work with webhooks. Most of them use json Files to customize the properties as described here:

https://developers.mattermost.com/integrate/incoming-webhooks/

It would be perfect, if we can choose "custom json" instead of "Teams" or "slack" and upload our own json file to send to the given webhook url.

alternative you add mattermost to the list. its not really different from slack integration.