A webhook as notification channel

[mvarendorff]

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Is your feature request related to a problem? Please describe.
Getting E-Mail notifications to work with a self-hosted setup is rather tedious and I do not have Telegram, however I do use Discord quite frequently. Discord allows sending messages through a webhook and lots of other platforms do too so this could also serve as workaround for other platforms until they are explicitly implemented.

Describe the solution you'd like
Allow setting up a webhook as notification channel. Required fields would be the URL, the body (which allows templating the content to allow it to be incorporated in a JSON structure for instance). Additionally it probably requires a request-method + content-type header field, shamelessly "inspired" by IFTTT's action:

Describe alternatives you've considered (optional)
I didn't come up with alternatives.

Additional context
I am willing to implement this, although I would probably need some guidance on where to start.