title: Gson SerializedName in a Kotlin interface
body: <p>Is there any way to use the gson SerializedName annotation in a Kotlin interface?</p>
  
  <pre><code>interface User {
     @SerializedName("user_id")
     val userId: String?
  }
  
  data class MainUser(override val userId: String? = null) : User
  </code></pre>
  
  <p>I know the annotation can be in the child class, but that's not what I'm looking for as there are many classes extending that interface.</p>
  
  <p><strong>Note 1:</strong>
  Using abstract or open class as a parent of a data class will solve this, but the new problem would be the overrided equals() method in the parent never get called (mainUser1 == mainUser2 never check the equls() method in abstract or open class User )</p>