[Security] Adding typehint

[ThomasLandauer]

...from https://symfony.com/doc/current/security/guard_authentication.html#the-guard-authenticator-methods

[javiereguiluz]

Thanks for this fix!

I've been checking the related interface and:

• In 4.4 there's no typehint: https://github.com/symfony/symfony/blob/e2f430dfb4c0c8cdde01ed111f4f0851e268ab5a/src/Symfony/Component/Security/Guard/AuthenticatorInterface.php#L140
• In 5.2 there's a typehint: https://github.com/symfony/symfony/blob/59f97179e8b669709edfdedd5f2e10d1ea3f3d01/src/Symfony/Component/Security/Guard/AuthenticatorInterface.php#L136

That's why we've merged this in 5.2 and 5.x but not in 4.4. Cheers!