This commit is introducing the following macro changes:
(1) Migrate NAS 5GS lib files to a new 5GS subfolder in NR_UE
and organize NAS 5GS lib messages into 5GMM and 5GSM subfolders.
A nested CMake structure was implemented and CMakeLists.txt were
added whenever necessary. This is effectively separating 5GS
libraries from LTE libraries.
(2) In order to simplify the separation of 5GS libs from LTE libs,
remaining dependencies from NAS LTE (e.g. nas_ue_task.c, ue-idx-standalone)
were removed in the process
(3) ExtendedProtocolDiscriminator.c/.h and SpareHalfOctet.c/.h were deleted
and affected types were replaced with simple integer types
(4) Cleanup: unused definitions and libs (e.g. LIB_5GNAS_GNB)
were removed
(5) Update doc/Doxyfile
(6) clang-formatted the migrated files in the process
(7) Update includes of headers in affected files
