Add async webhook that notifies about certain changes to attendees

[Jumpy-Squirrel]

In the configuration, allow adding a list of target URLs to send asynchronous webhooks to if an attendee changes in the following ways:

• due amount changed
• remaining dues changed
• due date changed
• status changed
• packages changed
• options changed
• flags changed
• nickname changed
• avatar URL changed (new field taken from JWT), for now can only detect change if user is seen

The webhook should be a POST request, and only contain the information listed, but in an extensible format. Especially must consider that packages will receive a count.

For now, we do not implement exponential back-off, but the hook should be sent in a goroutine to not hold up requests.

Hook requests should send request IDs but no authorization. We expect the URL to include a secret part.

The first recipient of this webhook will be reg-room-service, to be able to remove cancelled attendees from groups and rooms.