NAS Service Request enc/dec implementation

NAS Service Request:

• encoding
• decoding
• unit test

Related to #852

openair3/NAS/COMMON/EMM/MSG/CMakeLists.txt

+add_library(fgs_lib OBJECT
+            fgs_service_request.c
+)
+
+target_link_libraries(fgs_lib PUBLIC fgs_ies_lib)
+
+target_include_directories(fgs_lib PUBLIC
+                           ${CMAKE_SOURCE_DIR}
+                           ${CMAKE_CURRENT_SOURCE_DIR}
+)