XNAP: Introduce XnAP library framework and add encode/decode, unit test for Xn Setup Request

- 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>

openair3/UTILS/conversions.h

@@ -497,6 +497,16 @@ do {                                                    \
     (bITsTRING)->bits_unused = 4;                       \
 } while(0)
 
+#define MACRO_BIT_STRING_TO_GNB_ID(bITsTRING, oUT)          \
+do {                                                        \
+    uint8_t *_buf = (bITsTRING)->buf;                       \
+    (oUT) =                                                 \
+        ((uint32_t)_buf[0] << 20) |                         \
+        ((uint32_t)_buf[1] << 12) |                         \
+        ((uint32_t)_buf[2] << 4)  |                         \
+        (((uint32_t)_buf[3] & 0xF0) >> 4);                  \
+} while (0)
+
 /* TS 36.413 v10.9.0 section 9.2.1.38:
  * E-UTRAN CGI/Cell Identity
  * The leftmost bits of the Cell