Add the shopify webhook controller
user
```ruby
module Spree
  module Retail
    class ShopifyHookController < ApplicationController
      before_filter :verify_request_authenticity

      private

      def verify_request_authenticity
        data = request.body.read

        head :unauthorized unless request_authentified?(data, Spree::Retail::Config.shopify_shared_secret)
      end

      def request_authentified?(data, secret)
        hmac_header = env['HTTP_X_SHOPIFY_HMAC_SHA256']
        digest = OpenSSL::Digest::Digest.new('sha256')
        calculated_hmac = Base64.encode64(OpenSSL::HMAC.digest(digest, secret, data)).strip
        calculated_hmac == hmac_header
      end

      def json_body
        @_json_body ||= JSON.parse(request.body.read)
      end
    end
  end
end
```
assistant