title: add unique constraint in room database to multiple column
body: <p>I have one entity in room</p>
  
  <pre><code>@Entity(foreignKeys ={
          @ForeignKey(entity = Label.class, parentColumns = "_id", childColumns = "labelId", onDelete = CASCADE),
          @ForeignKey(entity = Task.class, parentColumns = "_id", childColumns = "taskId", onDelete = CASCADE)
  })
  public class LabelOfTask extends Data{
      @ColumnInfo(name = "labelId")
      private Integer labelId;
      @ColumnInfo(name = "taskId")
      private Integer taskId;
  }
  </code></pre>
  
  <p>sql syntax of this entity is as below</p>
  
  <pre><code>CREATE TABLE `LabelOfTask` (
      `_id` INTEGER PRIMARY KEY AUTOINCREMENT,
       `labelId` INTEGER,
       `taskId` INTEGER,
       FOREIGN KEY(`labelId`) REFERENCES `Label`(`_id`) ON UPDATE NO ACTION ON DELETE CASCADE ,
       FOREIGN KEY(`taskId`) REFERENCES `Task`(`_id`) ON UPDATE NO ACTION ON DELETE CASCADE
   );
  </code></pre>
  
  <p>but what change or annotation I need to add in entity class if I want to append below constraint to the auto generated sql schema of the table</p>
  
  <pre><code>unique (labelId, taskId)
  </code></pre>
  
  <p>Ultimately I want to make combination of labelId and taskId unique in a table(or entity of room) using room library.</p>