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>

doc/SW_archi.md

@@ -329,7 +329,7 @@ To manage UE connections, `nr_pdcp_add_srbs()` is employed for adding UE SRBs in
 
 ## PDCP Tx flow
 
-On the Tx side (downlink in gNB), the entry functions `nr_pdcp_data_req_drb()` and `nr_pdcp_data_req_srb()` are called by the upper layer. The upper layer could be GTP or a PDCP internal thread like `enb_tun_read_thread()`, which reads directly from the Linux socket if the 3GPP core implementation is skipped. The PDCP internals for `nr_pdcp_data_req_srb()` and `nr_pdcp_data_req_drb()` are thread-safe. Within these functions, the PDCP manager protects access to the SDU receiving function of PDCP (`recv_sdu()` callback, corresponding to `nr_pdcp_entity_recv_pdu()` for DRBs) using mutex. When necessary, the PDCP layer pushes this data to RLC by calling `rlc_data_req()`.
+On the Tx side (downlink in gNB), the entry functions `nr_pdcp_data_req_drb()` and `nr_pdcp_data_req_srb()` are called by the upper layer. The upper layer could be GTP or a PDCP internal thread like `gnb_tun_read_thread()`, which reads directly from the Linux socket if the 3GPP core implementation is skipped. The PDCP internals for `nr_pdcp_data_req_srb()` and `nr_pdcp_data_req_drb()` are thread-safe. Within these functions, the PDCP manager protects access to the SDU receiving function of PDCP (`recv_sdu()` callback, corresponding to `nr_pdcp_entity_recv_pdu()` for DRBs) using mutex. When necessary, the PDCP layer pushes this data to RLC by calling `rlc_data_req()`.
 
 ## PDCP Rx flow