title: Mapstruct : map field conditionally or ignore
body: <p>I have a class something like this : </p>
  
  <pre><code>class StudentDTO {
      String name;
      Integer rollNo;
      List&lt;CourseDTO&gt; coursesTaken;
      Boolean isFailed;
      List&lt;CourseDTO&gt; failedCourses;
  }
  </code></pre>
  
  <p>I want to map <code>failedCourses</code> List from StudentDTO to Student only if the flag <code>isFailed</code> is true, else ignore the field, but without using default implementation in interface. Is there any annotation/paramater in mapstruct that can help me? I've tried using <code>expression</code> but can't make it work. </p>