feat: Support Shoutrrr Generic Webhook

[ThinkChaos]

Is your feature request related to a problem? Please describe.
Shoutrrr supports arbitrary services via the Generic Webhook notifier.

Describe the solution you'd like
It would be great to expose this in Argus to support any possible notification service.

Describe alternatives you've considered
N/A

Additional context
I use a custom notifier and generally this works fine, but is not possible with Argus at the moment.

[ThinkChaos]

Figured out I can do it without explicit support via type: shoutrrr.
Would still be nice to have type: generic support IMO.