Introduce dedicated Services getDefaultSource() and setDefaultSource(int $sourceId)

as a part of newly created Module InventoryCatalog (ticket #96 in the scope of current Story).

These services supposed to be used on Product Management form processing in Admin UI .

Please, read Design Document covering Business Value of this story implementation, as well as how we recommend implementing these services - https://github.com/magento-engcom/msi/wiki/The-Concept-of-Default-Source-and-Domain-Driven-Design