Stock item indexer

[naydav]

Introduce new Inventory index, which will produce and fill the stock_item table with StockItems based on Source to Stock assignment and Inventory on Sources.

Need to create an interface like as StockItemIndexer, StockItemManager in module Inventory
which would be used during the StockItem re-indexation to create StockItem entities

StockItem is aggregated data by Sources Qty which are assigned to the concrete Stock.

Think about how we will use this index, it will direct join, or we need to provide Api interface

Technical Design Document https://github.com/magento-engcom/magento2/wiki/New-Indexer-for-StockItems