拥有对 CertificateHandler 对象的引用，该对象可管理此 UnityWebRequest 的证书验证。
Setting this property to null makes the platform use the default certificate validation, which will validate certificates against a root certificate authority store (most commonly Operating System store).
并非所有品台都支持证书验证回调。请参阅 CertificateHandler 了解受支持的平台列表。
This property cannot be changed after calling SendWebRequest.
默认值：null。