Add service in Magento Backoffice to retrieve all prices related data for initial sync data

AC:

• raw prices of simple products should be returned (without relying on the existing price indexer)
  • special price, special price date
  • tier prices
• custom options prices
• product links prices. (downloadable)
• product/options relations
  • for composite products
  • for custom options

repo: <https://github.com/magento/commerce-data-export]

Tech details:

• list of all price-related events Price change notification #443 (comment) (only part of them should be honored for synchronization)
• utilize the same approach (data providers) for raw price collecting data which used in Price change notification #443
  • refactor message structure if needed to have similar response for price change event and synchronisation response
• "id" field in Request/Response object is a string that represents a specific entity, e.g.
  • Introduce Interface to build product id (refactor existing providers, e.g. Magento\CatalogDataExporter\Model\Provider\Product\ProductOptions\OptionValueUid)
  • for product: regular Magento Product ID returned, e.g. "42"
  • for custom option: "option/<customoption_id]/[custom_option_valueid]"
• use batching for return prices (generators usage is preferred)
   

Proposed response 

{
  "prices":[
    {
      "meta":{
        "code":"price",
        "w":0,
        "cg":1
      },
      "data": [
        {
          "id":"71",
          "value":"12.0"
        },
        // ...
      ]
    },   
    {
      "meta":{
        "code":"tier_price",
        "w":0,
        "cg":1
      },
      "data": [
        {
          "id":"71",
          "value":"12.0",
          "qty": 2
        }
      ]
    }
  >
}