The request
Response object to be used to redirect the client to the authentication server
The OAuth settings that apply to this login attempt
(Optional) An array of identifiers specifying the scope of the authorization requested.
true if a OAuth session was obtained and false if no OAuth session is present and a redirect to an OAuth provider will happen.
Perform OAuth login using the given _settings
The route mapped to this method should normally match the redirectUri set on the settings. If multiple providers are to be supported, there should be a different route for each provider, all mapped to this method, but with different settings.
If the request looks like a redirect back from the authentication server, settings.userSession is called to obtain an OAuthSession.
Otherwise, the user agent is redirected to the authentication server.