Notification displayed when the validation fails.
Can be defined as a static string or a callback function returning a string.
When validate references one of the predefined rule keys, this
property overrides that rule's built-in notification.
Localized notifications for predefined rules configured directly by key
can be customized through lang.validationNotifications.
Validation logic for the rule.
Use a built-in rule key to reuse one of the predefined validators, or provide a custom callback function.
Definition of the validation rule that should contain validate method and validation notification.