title: Is there a programmatic way to change user agent in Cypress.io?
body: <p>I have some ad calls that are only made on mobile devices.  In Chrome, I can use Device Mode and simulate a mobile device, and the resulting ad call from the server is correctly tailored to mobile.  I'm not sure how Chrome does this, except possibly by sending a different user agent.</p>
  
  <p>In the Cypress.io documentation, it says the user agent can be changed in the configuration file (Cypress.json).  But, I need to run a test for a desktop viewport and then a mobile viewport with a mobile user agent.  Is there a way to change the user agent programmatically?  </p>