title: Laravel: Validate field only when the value changed
body: <p>Some form fields in my laravel 5.5 application have validation rules that run against a remote API and take quite some time. I would thus only want to run these expensive checks when the field value changes (is different from the value currently stored in the model).</p>
  
  <p>Is there something that implements this already, e.g. as a rule similar to <code>sometimes</code>?</p>
  
  <p>I would image it like this: <code>only_changed|expensive_validation|expensive_validation2</code>. The latter rules would only be executed when the field value has changed.</p>