Authenticate a user.

The first argument is a JSON object containing information for authenticating the user. What this actually contains depends on the specific implementation. In the case of a simple username/password based authentication it is likely to contain a JSON object with the following structure:

   {
     "username": "tim",
     "password": "mypassword"
   }
 

For other types of authentication it contain different information - for example a JWT token or OAuth bearer token.

If the user is successfully authenticated a User object is passed to the handler in an async result. The user object can then be used for authorisation.