Introduce API interfaces for Assigning Stocks to Sales channels

[maghamed]

• New API for assigning Stocks to Sales Channels should be introduced
• Resolver which returns Stock_id based on Sales channel data (type and code) should be introduced
• Add extension attribute SalesChannels to the StockInterface

Read more details in the scope of this story - https://github.com/magento-engcom/msi/wiki/Stocks-to-Sales-Channel-Mapping

[naydav]

#173
#150