Pinput should expose FormField's autovalidateMode property

[AmmarAyman108]

Title: Add support for autovalidateMode

Description:

It would be great if Pinput supported Flutter's FormField autovalidateMode property.

Currently, validation messages are not always shown immediately when the field contains incomplete input unless focus is lost or FormState.validate() is called manually.

Expected behavior:

Pinput(
  autovalidateMode: AutovalidateMode.onUserInteraction,
  validator: (value) {
    if (value == null || value.length != 4) {
      return 'Invalid OTP';
    }
    return null;
  },
)

Benefits:

• Consistent behavior with Flutter FormField widgets
• Better UX for OTP validation
• Less need for manual focus handling
• Easier integration with existing forms

Thank you for the great package.

[AmmarAyman108]

@Tkko