Support for threads in the Discord notification

[emipa606]

Is your feature request related to a problem? Please describe.

Since Discord added threads-channels it has become a great way to organize notifications from various services.
The webhook for this is:
https://discord.com/api/webhooks/WEBHOOKID/TOKEN?thread_id=THREADID
Using variations of this in the WATCHTOWER_NOTIFICATION_URL= like
discord://TOKEN@WEBHOOKID?THREADID or similar just gives an error about
... is not a valid config key

Describe the solution you'd like

I would like to have the option to add the thread-id to be able to post notification to a Discord-thread.

Describe alternatives you've considered

.

Additional context

No response

[github-actions]

Hi there! 👋🏼 As you're new to this repo, we'd like to suggest that you read our code of conduct as well as our contribution guidelines. Thanks a bunch for opening your first issue! 🙏