S1AP eNB/MME DIRECT INFORMATION TRANSFER not implemented

[pespin]

Issue Description

I just looked at current srsRAN_4G master (fa56836b14dc6ad7ce0c3484a1944ebe2cdbe63b) and as far as I can tell, srsENB is not implementing:

• Tx of eNB DIRECT INFORMATION TRANSFER (3GPP TS 36.413 sec 8.13)
• Rx of MME DIRECT INFORMATION TRANSFER (3GPP TS 36.413 sec 8.14)

Those messages are generated by the eNB eg. when the UE requests information (SI) about some GERAN neighbor cell, aka NACC procedure.

I am currently implementing the handling/transmitting of those messages in open5gs-mmed (https://github.com/open5gs/open5gs/compare/main...pespin:open5gs:pespin/gn), as well as adding a Gn interface so that it can talk to pre-rel8-SGSNs (such as osmo-sgsn). Related tickets: https://github.com/open5gs/open5gs/issues/2410 https://osmocom.org/issues/5759

We expect to have TTCN3 tests validating the scenario against open5gs-mmed soon in https://jenkins.osmocom.org/jenkins/view/TTCN3/job/ttcn3-mme-test-ogs/