Support for Mattermost Alerts

[myugan]

Description

Slack is commonly used for sending alerts, and many applications support it either through custom webhooks or dedicated Slack integrations. Since Mattermost supports Slack-compatible webhooks, it can often utilize the same integrations meant for Slack. I would like to request to support Mattermost for alerts either by allowing configuration of custom webhooks or by ensuring that the existing Slack integration is compatible with Mattermost's webhook system.