POC NSM declarative EPC user plane with separate routed networks

Based on the work to [create a routed network version of the K8s-based declarative EPC user plane](https://github.com/cncf/cnf-testbed/milestone/56)

Using NSM to dynamically add network interfaces, provide network service discovery and connectivity for the Go-GTP EPC example

Start by reviewing milestone 56 and the Go-GTP EPC example referenced in the issue. The issue does not name files or tests; completion would require demonstrating that NSM dynamically adds network interfaces and provides network service discovery and connectivity for the EPC user plane.

Written by the indexing model from the issue text.