Refactor TUN Interface Management and Move to SDAP Layer

This commit refactors the TUN interface management by moving its creation
and configuration from the PDCP layer to the SDAP layer for UE.

Key changes include:

- Introduced `tun_generate_ue_ifname` and `tun_destroy` functions for better
  TUN interface lifecycle management.
- Added support for creating and destroying UE-specific TUN interfaces in SDAP.
- Refactored `nr_pdcp_module_init` to remove TUN-related logic and moved it
  to SDAP.
- Updated NAS and SDAP layers to handle PDU session-specific TUN interface
  creation and configuration.
- Enhanced SDAP entity management to include TUN socket handling and thread
  lifecycle.
- Updated NAS message handling to initialize default PDU session TUN
  interfaces in NOS1 mode.
- Use a single SDAP thread function for gNB and UE
- Allow noS1 mode also in SA on gNB

These changes improve modularity by isolating TUN interface management within
the SDAP layer, aligning with the 3GPP architecture, and simplifying the PDCP
layer.

Co-Authored-By: Robert Schmidt <robert.schmidt@openairinterface.org>

openair2/COMMON/as_message.h

@@ -469,6 +469,10 @@ typedef ul_info_transfer_cnf_t dl_info_transfer_cnf_t;
  */
 typedef ul_info_transfer_ind_t dl_info_transfer_ind_t;
 
+typedef struct nas_nos1_msg_s {
+  int dummy; // This dummy element is to avoid CLANG warning: empty struct has size 0 in C, size 1 in C++
+} nas_nos1_msg_t;
+
 typedef struct nas_pdu_session_req_s {
   int pdusession_id;
   int pdusession_type;

openair2/COMMON/rrc_messages_def.h

@@ -79,6 +79,7 @@ MESSAGE_DEF(NR_NAS_CONN_ESTABLISH_IND, MESSAGE_PRIORITY_MED, nas_establish_ind_t
 MESSAGE_DEF(NR_NAS_CONN_RELEASE_IND,    MESSAGE_PRIORITY_MED,       NRNasConnReleaseInd,        nr_nas_conn_release_ind)
 MESSAGE_DEF(NAS_UPLINK_DATA_CNF, MESSAGE_PRIORITY_MED, ul_info_transfer_cnf_t, nas_ul_data_cnf)
 MESSAGE_DEF(NAS_DOWNLINK_DATA_IND, MESSAGE_PRIORITY_MED, dl_info_transfer_ind_t, nas_dl_data_ind)
+MESSAGE_DEF(NAS_INIT_NOS1_IF, MESSAGE_PRIORITY_MED, nas_nos1_msg_t, nas_init_nos1_if)
 
 // xNB: realtime -> RRC messages
 MESSAGE_DEF(RRC_SUBFRAME_PROCESS,       MESSAGE_PRIORITY_MED,       RrcSubframeProcess,         rrc_subframe_process)