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>

common/utils/tun_if.h

@@ -34,6 +34,7 @@
  * \param[in] instance_id unique instance number
  */
 int tun_generate_ifname(char *ifname, const char *ifprefix, int instance_id);
+int tun_generate_ue_ifname(char *ifname, int instance_id, int pdu_session_id);
 
 /*!
  * \brief This function initializes the TUN interface
@@ -73,4 +74,18 @@ bool tun_config(const char* ifname, const char *ipv4, const char *ipv6);
  */
 void setup_ue_ipv4_route(const char* ifname, int instance_id, const char *ipv4);
 
+/*!
+ * \brief This function allocates a TUN interface
+ * \param[in] dev name of the interface
+ * \return file descriptor of the allocated interface
+ */
+int tun_alloc(const char *dev);
+
+/*!
+ * \brief This function destroys the TUN interface
+ * \param[in] dev name of the interface
+ * \param[in] fd file descriptor of the allocated interface
+ */
+void tun_destroy(const char *dev, int fd);
+
 #endif /*TUN_IF_H_*/