Enhance UE identity management in Initial UE Message and other NGAP improvements

This MR improves the Initial UE Message procedures by supporting simultaneous presence of both 5G-S-TMSI and GUAMI. Previously, only one identity could be used, leading to potential AMF selection failures. Now the presenceMask allows both identities, The AMF selection is prioritizing GUAMI first, then falling back to 5G-S-TMSI, and finally using PLMN or highest capacity.

Also, fix handling of selected PLMN identity in NGAP.

Refactoring:

• Select the AMF based on (1) absence of UE Identity (2) Presence of GUAMI (3) Presence of 5G-S-TMSI
• Also refactored the AMF selection logic for better readability and reliability.
• Refactor and cleanup rrc_gNB_send_NGAP_NAS_FIRST_REQ
• Replace ngap_pdu_t with byte_array_t
• Add guami_t to ngran_types.h: refactor NGAP and RRC code to use one common GUAMI type instead of two identical ones

Also:

• Bugfix in ngap_gNB_handle_ng_setup_response: use correct count for PLMNSupportList

Closes issue #801 (closed)