title: class-validator validate union type
body: <p>I have a <a href="https://mongoosejs.com/docs/discriminators.html" rel="noreferrer">mongoose discriminator schema</a>, which mean the data will be different according to one of the attributes.</p>
  
  <pre class="lang-js prettyprint-override"><code>class Feature {
    name: string
    option: ColorFeature|SizeFeature
  }
  
  class ColorFeature {
    kind: 'color'
    color: string
  }
  
  class SizeFeature {
    kind: 'size'
    size: number
  }
  </code></pre>
  
  <p>What is the correct way to validate the <code>Feature</code> class so that it only accepts 2 different kinds?</p>