E1AP: Add PDU Session Resource To Remove List to Bearer Context Modification enc/dec

According to TS 38.463 - clause 9.2.2.4 and 9.3.3.12.
Also, fix response message by encoding choice IE only
with presence of modified PDU sessions.

Co-authored-by: rmagueta <rmagueta@allbesmart.pt>

openair2/COMMON/e1ap_messages_types.h

@@ -475,6 +475,14 @@ typedef struct pdu_session_to_mod_s {
   DRB_nGRAN_to_mod_t DRBnGRanModList[E1AP_MAX_NUM_DRBS];
 } pdu_session_to_mod_t;
 
+/** PDU Session Resource To Remove List (3GPP TS 38.463 clause 9.3.3.12) */
+typedef struct {
+  // PDU Session ID (M)
+  long sessionId;
+  // Cause (O)
+  e1ap_cause_t cause;
+} pdu_session_to_remove_t;
+
 /**
  * Bearer Context Setup Request message, clause 9.2.2.1 of 3GPP TS 38.463
  */
@@ -534,6 +542,9 @@ typedef struct e1ap_bearer_mod_req_s {
   // NG-RAN PDU Session Resource To Modify List (O)
   int numPDUSessionsMod;
   pdu_session_to_mod_t pduSessionMod[E1AP_MAX_NUM_PDU_SESSIONS];
+  // NG-RAN PDU Session Resource To Remove List (O)
+  int numPDUSessionsRem;
+  pdu_session_to_remove_t pduSessionRem[E1AP_MAX_NUM_PDU_SESSIONS];
 } e1ap_bearer_mod_req_t;
 
 typedef struct e1ap_bearer_release_cmd_s {