feat: support configuring local IP address for N9 interface with N3 address by default - #585

Until now, eupf only allowed configuring the local IP address for the N3
interface, which actually ended up being used for both N3 and N9 (GTP-U
forwarding towards other uplink UPFs).

However, that limitation becomes a problem under some production
scenarios. Some users require running a UPF to forward GTPv1U traffic from
network A to network B and viceversa, where those 2 networks are not reachable
to each other, only through the UPF host.
This happens for instance when cellular operators don't control/own the
entirety of their networks, e.g. a network which runs their NAT under a
satellite link with private addresses (eg VPN) which end up at a single host.

As a result, the UPF requires managing at least 2 separate local IP addresses
for GTP-U traffic: One (usually private) IP address towards the RAN, and one
(usually public, or another private network) IP address towards the Core.
As a result, when a PFCP client (an SMF, or osmo-hnbgw, or osmo-s1gw to mention
some other examples) tries to setup a session to forward GTPU traffic from
network A (RAN) to network B (Core), will result in eUPF forwarding the GTP-U
packet to the Core but using the RAN configured IP as the GTPU packet local IP
address.
Hence, either local routing will be wrong, or the Core will drop the received
packet, or won't know how to route traffic back to it.

This patch allows eupf to be used co-located with 3G osmo-hnbgw [1] and
4G osmo-s1gw [2] under the mentioned network setups, both supporting PFCP to
manage a UPF to forward data plane also in GTP1U protocol.

[1] https://osmocom.org/projects/osmohnbgw
[2] https://gitea.osmocom.org/erlang/osmo-s1gw

Related: #577