Add mixed return type for key and current methods

[DavidePastore]

This is the warning that I see in the logs when using the client:

User Deprecated: Method "Iterator::key()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "Akeneo\Pim\ApiClient\Pagination\ResourceCursor" now to avoid errors or add an explicit @return annotation to suppress this message. {"exception":"[object] (ErrorException(code: 0): User Deprecated: Method \"Iterator::key()\" might add \"mixed\" as a native return type declaration in the future. Do the same in implementation \"Akeneo\\Pim\\ApiClient\\Pagination\\ResourceCursor\" now to avoid errors or add an explicit @return annotation to suppress this message. at /var/www/app/vendor/symfony/error-handler/DebugClassLoader.php:341)"} {"containerName":""}

This PR enforces the return types of key and current methods.