- Add initial XnAP library structure under openair2/XNAP/lib
  - Implement common XnAP helper utilities (xnap_lib_common)
  - Add gNB interface management module for Xn Setup procedures
  - Introduce XnAP message type definitions in xnap_messages_types.h
  - Integrate XnAP library into build system (CMakeLists.txt)
  - Add standalone XnAP library unit test (xnap_lib_test)
  - Implement encoder, decoder + test for Xn Setup Request
     Xn Setup Request (3GPP TS 38.423v16.2.0 §9.1.3.1)
	- Global NG-RAN Node ID (M)
	- TAI Support List (M)
	- AMF Region Information (M)
  - Extend conversions.h with MACRO_BIT_STRING_TO_GNB_ID utility
Co-authored-by: Sreeshma Shiv <sreeshmau@iisc.ac.in>