Requests a URL which authenticates an in-game browser for store check-out,
and then redirects to the specified URL. As long as the in-game browser
accepts and handles session cookies, Steam microtransaction checkout pages
will automatically recognize the user instead of presenting a login page.
The result of this API call will be a StoreAuthURLResponse_t callback.
NOTE: The URL has a very short lifetime to prevent history-snooping attacks,
so you should only call this API when you are about to launch the browser,
or else immediately navigate to the result URL using a hidden browser window.
NOTE 2: The resulting authorization cookie has an expiration time of one day,
so it would be a good idea to request and visit a new auth URL every 12 hours.