Add runtime configuration class

[rafalcymerys]

Currently, Spree::Config, Spree::Backend::Config etc. use Spree::Preferences::Configuration class as a base. The problem with Spree::Preferences::Configuration is that it persists every preference in the database, so that they can be e.g. changed via the UI in the admin panel.

This causes some major issues with the initialization process. As an example, Spree::Backend defines a preference called admin_path that's used to define the root for its routes. The value needs to be known at boot time, and cannot be dynamically changed. Similar case with Spree::Auth::Devise, where certain Devise features are enabled at boot time, based on configuration.

Because of that, we should introduce an additional type of Configuration - RuntimeConfiguration, that's not persisted and that should be only defined in an initializer. This will allow to run the configuration step before after_initialize, and will work regardless of whether the database connection is ready.

This PR implements a draft of such solution that could be later reused in other gems and extensions.

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[mateus-po]

Looks great:))

[tomdonarski]

Good call with being independent from db connection.
I'd need to have yet another sit on this class, as it's a bit hard to comprehend (the test helps to get the gist of it, so thank you for adding that).
On the other hand boot- and config-related things are kind of special ad not changed that often, so that extra bit of complexity is understandable.