Move api_configuration from /app to /lib

It has nothing to do with Rails anymore and follows the style
of what we do in core

api/lib/spree/api/config.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+require 'spree/api_configuration'
+
+module Spree
+ module Api
+ Config = Spree::ApiConfiguration.new
+ end
+end

api/lib/spree/api/engine.rb

@@ -1,16 +1,17 @@
 # frozen_string_literal: true
 
 require 'rails/engine'
+require 'spree/api/config'
 
 module Spree
  module Api
  class Engine < Rails::Engine
  isolate_namespace Spree
  engine_name 'spree_api'
 
- initializer "spree.api.environment", before: :load_config_initializers do |_app|
-  Spree::Api::Config = Spree::ApiConfiguration.new
- end
+ # Leave initializer empty for backwards-compatability. Other apps
+ # might still rely on this event.
+ initializer "spree.api.environment", before: :load_config_initializers do; end
  end
  end
 end