title: How to allow any other key in Joi
body: <p>I have a simple requirement. I tried to search on the internet as well as documentation but failed. <br>
  So here is what I want to achieve:</p>
  
  <p>I have a schema:</p>
  
  <pre><code>const schema = Joi.object().keys({
    a: Joi.string().required(),
    b: Joi.string().required()
  });
  </code></pre>
  
  <p>Now, How do I configure it such that any other key in the object would be allowed? <br><br>
  With this schema, it only allows two keys <code>a</code> and <code>b</code>. If I pass any other key, say, <code>c</code>, it throws an error saying that <code>c</code> is not allowed.</p>