feat: extend C box API with new user methods

Introduce new methods:

1. box_user_id_by_name - get the user identifier by name;
2. box_effective_user_id - get current effective user
   identifier;
3. box_session_user_id - get current session user identifier;
4. box_session_su - change current session user;

NO_DOC=picodata internal patch
NO_CHANGELOG=picodata internal patch
NO_TEST=picodata internal patch