cfg: separate PFCP Node ID from PFCP local address
The PFCP protocol features a Node ID, which can be used to uniquely identify
PFCP endpoints across different interfaces. The Node ID can be a fully-qualified
domain name (FQDN), or IP address. Currently, osmo-upf has no support for
using an FQDN as Node ID, and requires the 'local-addr' to be configured, which
it uses as Node ID.
Instead, we should separate the concepts of which local interface to use from the Node ID sent in-band.
pfcp_endpoint.h already has separate fields in osmo_pfcp_endpoint_cfg for local_addr and local_node_id, but osmo-upf only features a pfcp / local-addr config option.