title: How to generate column update option in model create net core 2.1?
body: <p>I use NetCore 2.1 SDK, database is Mysql (5.7.21) with Pomelo package connector (2.1 RC2).</p>
  
  <p>I have "updateAt" column in my database table, type datetime.</p>
  
  <p>I have scaffolded database where column was:</p>
  
  <pre><code>`UpdatedAt` datetime DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
  </code></pre>
  
  <p>after creating migration, the result is:</p>
  
  <pre><code>entity.Property(e =&gt; e.UpdatedAt)
                          .HasColumnType("datetime")
                          .HasDefaultValueSql("CURRENT_TIMESTAMP")
                          .ValueGeneratedOnAddOrUpdate(); 
  </code></pre>
  
  <p>When I run database update, column is missing "ON UPDATE" option.</p>
  
  <p>How could I change modelCreate to achieve this? </p>