The 5GMM library in openair3/NAS/NR_UE/5GS/5GMM/ contains the
all the definitions necessary to do the enc/dec of 5GMM IEs.
5GMM messages can share the same IEs, therefore they can use
common definitions and enc/dec functions.
The goal of this commit is to add common definitions to new shared
5GMM library files. The consumers of fgmm_lib are the functions
defined in the openair3/NAS/NR_UE/5GS/5GMM/ library.