Support for Mattermost to override the default channel

[beheersp]

Describe the feature request

When using a Mattermost webhook we want to override the channel the message posts in.
In our case we got many channels and want to use a single webhook to post to all the different channels.

The channel override is supported in Mattermost as the extra channel parameter which is mentioned on this page:
https://developers.mattermost.com/integrate/webhooks/incoming/#parameters

For example with Gatus custom alert we override the channel but this laks the information of the 'Condition results' fields.

      custom:
        url: "https://mattermost.localhost/hooks/hook_id"
        method: "POST"
        body: |
          {
            "channel":"my-channel","attachments": [{"title": "Gatus","title_link": "https://gatus-mattermost.localhost","color": "danger","text": "An alert for [ENDPOINT_GROUP]/[ENDPOINT_NAME] has changed status to [ALERT_TRIGGERED_OR_RESOLVED]\n[ENDPOINT_URL]"}]
          }

For this Feature the Gatus alert could be extended by an extra optional attribute channel which contains the channel name:

      mattermost:
        webhook-url: my-webhook
        channel: my-channel

Why do you personally want this feature to be implemented?

This feature makes it possible to use one webhook for all channels.
This keeps the configuration simple because we don't have to create and administrate a webhook for each channel.

How long have you been using this project?

3 years

Additional information

No response