SDN-R: PNF registration

Updates are made for O-RAN-SC G-Release after the O-RAN Alliance has specified the o1NotifyPnfRegistration event for VES-domain "stndDefined".

The PNF registration data flow was simplified for demonstration purposes for the O-RAN-SC Amber release.  

The implementation on SDN-R was made in a way that it won't conflict with the complete 5G PNF Plug'n'Play please see: PNF PLUG and PLAY in R6 Frankfurt and 5G - PNF Plug and Play.