Introduce separate gNB-CU-UP ID in configuration

3GPP has the concept of a gNB-CU-UP ID and a (separate) gNB ID (for gNB,
CU-CP). This commit introduces the gNB-CU-UP as a separate ID that has to be
set in the configuration file when running as a CU-UP. For the
CU/monolithic gNB, it is optional (but needs to be the same as the
gNB-ID if specified).

The CU-UP ID is necessary, as some entities, e.g., the e2 agent, need to
signal both IDs, e.g., to reconcile a CU-CP and (multiple) CU-UP(s)
belonging together.

openair2/COMMON/e1ap_messages_types.h

@@ -87,6 +87,7 @@ typedef struct e1ap_setup_req_s {
 typedef struct e1ap_register_req_t {
   e1ap_setup_req_t setup_req;
   e1ap_net_config_t net_config;
+  uint32_t gnb_id; // unused in CU-UP, but might be necessary for some functionality, e.g., E2 agent
 } e1ap_register_req_t;
 
 typedef struct e1ap_setup_resp_s {