Handle IPv6/IPv4v6 PDU sessions

Handle IPv6 (and IPv4v6) PDU sessions by correctly instructing
nas_config() to set up the interface with provided IP addresses.
Note that as of now, IPv6 PDU sessions have not been fully tested.

Remove a useless assert at the gNB; to my understanding the gNB does not
care about the IP session type (at least for IP), as it just
transparently tunnels the IP packets from/to GTP.

I added a comment in the code where to change to request a PDU session
different from IPv4. Currently it is hardcoded to IPv4, and it seems
that a default OAI 5GC config would reject a PDU session type different
from IPv4.

openair2/RRC/NR/rrc_gNB_NGAP.c

@@ -310,7 +310,6 @@ static int decodePDUSessionResourceSetup(pdusession_t *session)
         /* mandatory PDUSessionType */
       case NGAP_ProtocolIE_ID_id_PDUSessionType:
         session->pdu_session_type = (uint8_t)pdusessionTransfer_ies->value.choice.PDUSessionType;
-        AssertFatal(session->pdu_session_type == PDUSessionType_ipv4 || session->pdu_session_type == PDUSessionType_ipv4v6, "To be developped: support not IPv4 sessions\n");
         break;
 
         /* optional SecurityIndication */