[🚀] User-customizable notification channels

[MarkL4YG]

Instead of the channel checking how to deliver a notification to a specific user, the user should be able to provide that information using a command that is controlled by the notification service.
Channels could provide a validation & verification method for the information and report whether or not the credentials have been accepted.
The information should then be stored in the user's corresponding profile using the channel identifier.
Multiple entries should be supported in some way. (Think of multiple targets using the same channel.)