Forward AMF UE NGAP ID to RRC on DL NAS Message

In the Initial Context Setup message, the NGAP forwards the AMF UE ID,
but not in the DL NAS Message, although 38.413 says, in $8.3.1.2 either

> The NG-RAN node shall use the AMF UE NGAP ID IE and RAN UE NGAP ID IE
> received in the INITIAL CONTEXT SETUP REQUEST message as identification
> of the logical connection

or in $8.6.2.2

> The NG-RAN node shall use the AMF UE NGAP ID IE and RAN UE NGAP ID IE
> received in the DOWNLINK NAS TRANSPORT message as identification of the
> logical connection

So forward in both cases for harmonization.

openair2/COMMON/ngap_messages_types.h

@@ -737,6 +737,8 @@ typedef struct ngap_ue_ctxt_modification_resp_s {
 typedef struct ngap_downlink_nas_s {
   /* UE id for initial connection to NGAP */
   uint32_t gNB_ue_ngap_id;
+  /* UE id at AMF */
+  uint64_t amf_ue_ngap_id;
   /* NAS pdu */
   byte_array_t nas_pdu;
 } ngap_downlink_nas_t;