Feature: update notification envelope structs to reflect latest draft updates

[rodonile]

TODO:

• contents is now mandatory -> adjust data modeling
• remove deprecated leaf name and notification variant (to verify if not needed anymore)

https://github.com/network-analytics/NetCalyx/blob/main/crates/udp-notif-pkt/src/notification.rs#L240-L244
https://datatracker.ietf.org/doc/html/draft-ietf-netconf-notif-envelope