pfcp: implement sending Network Instance IEs

Allow configuring specific Network Instance names for the Core and
Access sides, to send to the UPF, to allow the UPF to pick the proper
network interface to create GTP tunnels on.

Add VTY cfg 'hnbgw' / 'pfcp' / 'netinst (access|core) NAME' to allow
configuring Network Interface values to send in PFCP. These are "dotted"
domain name strings, as in APN.

Add these Network Interface names to the PFCP messages' detection as
well as forwarding rules, each one indicating the side that it is
detecting on or forwarding to.

This helps lift osmo-hnbgw's PFCP support out of lab situations to a
proper production scenario, where the core and access networks are in
separate subnets, with osmo-hnbgw + UPF as the gateway.

For example, in osmo-hnbgw, configure

  hnbgw
   pfcp
    netinst access my-ran
    netinst core my-core

and in osmo-upf, configure

  netinst
   add my-ran 10.9.8.7
   add my-core 1.2.3.4

In effect, all GTP tunnel endpoints towards the Access side will be
bound on 10.9.8.7, and all GTP tunnel endpoints towards the Core side
will be bound on 1.2.3.4.

Related: SYS#5895
Change-Id: Ief53dbfacf1645c32a07847d590c4884d4c8ca56
---
M include/osmocom/hnbgw/hnbgw.h
M src/osmo-hnbgw/hnbgw_vty.c
M src/osmo-hnbgw/ps_rab_fsm.c
M tests/pfcp_cfg.vty.with_pfcp
4 files changed, 104 insertions(+), 2 deletions(-)