title: Use message key in Kafka connect source connector
body: <p>I'm using the Kafka connect JDBC source connector to read from a view in a database and post it on kafka, it is working fine.</p>
  
  <p>My use case is that a user can create multiple objects and the order of the objects is important in my application. I would like to use the user id as the message key of all the messages I'm posting into the topic to maintain their order.</p>
  
  <p>My question is how can I define a message key in the Kafka connect source connector?</p>