It is not possible to load any library in the GL868 or adding any new API. It is only possible to write and execute Python scripts and existing AT Commands.
Sorry but we are repeating again and again the same things. I'll try to make it clear. - the function for AES128 is implemented in the TLS stack library. It is used during TLS connections (AT#SSLD, AT#SSLSECDATA) as described in the SSL/TLS User Guide - there isn't any AT command or python function that allows to use the library for other purpouses.