fetch
Implement this method to connect to AXP's authentication API to get a Json Web Token (JWT) to use for authenticating this client & user.
Typically the code to talk to AXP to get the JWT is implemented in your own web application that talks to AXP, and your app's implementation of this interface will perform a call to your web application.
If there is any sort of failure to get the JWT, return null here and the corresponding HTTP request will fail with an authentication error.