Support for Discord

[Aankhen]

I’d like to get notifications on Discord with argocd-notifications. I tried pointing the engine’s slack service at the Slack-compatible webhook endpoint, but according to the trace-level logs, this returned a 404 because the actual request URL had chat.postMessage appended to it. In any case, Discord doesn’t support exactly the same things as Slack, so it’ll only work as long as the notifier doesn’t use these:

We do not support Slack's channel, icon_emoji, mrkdwn, or mrkdwn_in properties.

The existing webhook interface is another option, but it would be nice to have the engine provide a higher-level interface like it does for many other services.

EDIT: I may have misunderstood the Slack– & GitHub-compatible endpoints. I think they’re for incoming webhooks from those services, and so irrelevant to this discussion.