Support for Discord

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](https://discord.com/developers/docs/resources/webhook#execute-slackcompatible-webhook), 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.

The issue names the engine’s Slack service, the existing webhook interface, and trace-level logs showing the request URL behavior; no files or tests are specified. Start by tracing how the Slack service constructs webhook URLs and how the webhook interface is exposed. Done means the engine provides a higher-level Discord notification path without relying on the listed Slack-only properties.

Written by the indexing model from the issue text.