Implement support for discord threads.

[CodyCline]

Please describe your feature request:

Currently, there's no support for streaming messages to discord threads. The only workaround I've found is using the custom option in provider-config.yaml and appending a thread_id parameter to the webhook url.

To stay consistent, the feature would be implemented similarly to how it was in slack; where it sends a generic http request to the webhook service instead of using the default shoutrrr service when discord_threads is set to true (see slack_api.go). All the user has to do is add to their configuration is discord_threads:true AND discord_thread_id: 00000... I'm open to input on the implementation details if there's a better approach. Let me know if you need more information, this was also briefly discussed on your discord server yesterday.

Describe the use case of this feature:

For people that use discord, it would give more control over where notifications are posted to. Threads are extremely helpful in organizing and de-cluttering message channels.