Support for specific threads in discord

[k4deng]

When using discord notifications, sending a webhook to a thread through shoutrrr is impossible. (Sending in a forum channel)

This is possible through the use of the thread_id query string param, but I cannot seem to get it figured out through the generic service in shoutrrr, and it is not implemented in the discord service.

I would like to see this be implemented in the discord service.