[FEATURE] Add path to notification payload

[aroberts]

Is your feature request related to a problem? Please describe. The new notification system is great! For my purposes, I'm hoping to use them to link this project into a larger media system, and it's crucial to be able to use the notifications as a way to trigger downstream processing and file management when a download completes. Key to this is the path of the file, which as far as I can tell, is absent from the payload.

Describe the solution you'd like I'd be satisfied here with any machine-parsable representation of the path (file_obj.path is plenty, I think it's reasonable to have the consumer connect it to the deployed path) present in the download_complete notification. While investigating this, I noticed that the body of the notification POST is not very well-structured. If it were me, I'd move this to a JSON payload for ease of consumption. I could also see the utility in allowing users to template their own payload in the settings, though of course that's much more to build.

Additional context @Tzahi12345 if you're interested in this feature but don't have time to do it, just let me know what format I should follow for the body and I'll add it and submit a PR.