Integration of Symfony custom's user checker

[soufianZantar]

To check if my users in DB can login to the SAML or not, i use a custom symfony user checker https://symfony.com/doc/current/security/user_checkers.html. Today i can't config OneloginSamlBundle to add my user checker to Saml provider like GuardAuthenticationProvider and other providers that i use.

This PR will permits to integrate a user checker (implements Symfony UserCheckerInterface) in saml provider if it's configured like that :

     users_saml:
             saml:
                check_path: saml_acs
                login_path: saml_login
                failure_path: '/saml/error'
                default_target_path: '/agent'
                user_factory: saml_user_factory
                user_checker: App\Security\Checker\UserChecker