E1AP: add DRB To Remove list to E1 Bearer Context Modification lib

Add DRB To Remove List IE to PDU Session To Modify list, which is sent
in the Bearer Context Modification request. This allows the CUCP to
inform the CUUP about DRBs to be released in a specific PDU session
during a MODIFY procedure.

Changes:

* Add struct definition
* Add enc/dec/eq/cp func
* Add test

openair2/COMMON/e1ap_messages_types.h

@@ -409,6 +409,12 @@ typedef struct DRB_nGRAN_to_modify_s {
   qos_flow_to_setup_t qosFlows[E1AP_MAX_NUM_QOS_FLOWS];
 } DRB_nGRAN_to_mod_t;
 
+/* DRB To Remove Item (NG-RAN) clause 9.3.1.11 */
+typedef struct {
+  // DRB ID (M)
+  long id;
+} drb_to_remove_t;
+
 typedef enum e1ap_indication_e {
   SECURITY_REQUIRED = 0,
   SECURITY_PREFERRED,
@@ -472,6 +478,9 @@ typedef struct pdu_session_to_mod_s {
   long numDRB2Modify;
   // DRB To Modify Item (1..<E1AP_MAX_NUM_DRBS>)
   DRB_nGRAN_to_mod_t DRBnGRanModList[E1AP_MAX_NUM_DRBS];
+  // DRB To Remove List (0..maxnoofDRBs)
+  int n_drb_to_remove;
+  drb_to_remove_t drbs_to_remove[E1AP_MAX_NUM_DRBS];
 } pdu_session_to_mod_t;
 
 /** PDU Session Resource To Remove List (3GPP TS 38.463 clause 9.3.3.12) */