Move AuthConfiguration out of autoloading paths

The class is loaded during initialization and, beyond having no point
in being autoloaded, it generates an explicit warning in Rails 6, and
potential problems in prior versions.

Here's the Rails 6 warning:

```
DEPRECATION WARNING: Initialization autoloaded the constant Spree::AuthConfiguration.

Being able to do this is deprecated. Autoloading during initialization is going
to be an error condition in future versions of Rails.

Reloading does not reboot the application, and therefore code executed during
initialization does not run again. So, if you reload Spree::AuthConfiguration, for example,
the expected changes won't be reflected in that stale Class object.

This autoloaded constant has been unloaded.

Please, check the "Autoloading and Reloading Constants" guide for solutions.
 (called from <top (required)> at /Users/elia/Code/Nebulab/olx-ecommerce/config/environment.rb:5)
```

Author: elia

lib/spree/auth/engine.rb

@@ -12,6 +12,8 @@ class Engine < Rails::Engine
       engine_name 'solidus_auth'
 
       initializer "spree.auth.environment", before: :load_config_initializers do |_app|
+        require 'spree/auth_configuration'
+
         Spree::Auth::Config = Spree::AuthConfiguration.new
       end