refactor: have smaller main code by restructuring in new dedicated C files

Group gta-api functions according to their purpose in decicated C files like for example:
- management functions: gta_identifier_assign(), gta_identifier_enumerate(), gta_personality_...()
- data protection functions: gta_authenticate_data_detached() gta_seal_data(), gta_unseal_data(), ...
- channel protection functions: ...
- access control functions: ...