title: How to add add SSL .cert to chopper client?
body: <p>In flutter I try to create with chopper a request to a backend url.</p>
  <p>To my <code>ChopperClient</code> I try to provide a TLS certificate. I have the certificate
  as a file tls.cert</p>
  <p>How do I add this to my chopper requests?</p>