box: add stub functions to init extra security features

This commit adds function stubs security_init and security_free that
are called after auth_init and before auth_free, respectively. We'll
define them in the EE repository along with the enabling macros:
ENABLE_SECURITY and SECURITY_SOURCES. We'll start with adding extra
authentication methods in security_init. Later on, we'll implement more
security features, like password strength enforcement or authentication
delay.

Needed for https://github.com/tarantool/tarantool-ee/issues/295

NO_DOC=internal
NO_TEST=internal
NO_CHANGELOG=internal