socialite

Move auth url parameters to own method.

This makes it a bit easier to add any additional parameters needed for building
the auth url in a provider. For example, in Facebook you could often have the need to send the
"auth_type" parameter for forcing rerequests and reauthentications:

    // FacebookProvider

    protected function getCodeRequestParameters($state)
    {
        return array_merge(parent::getCodeRequestParameters($state), ['auth_type' => 'rerequest']);
    }