Source to Stock Mapping UI and API

[vrann]

Create the UI, backend logic, and the API to manage the Source mapping to the Stock according to the HLD https://github.com/magento-engcom/magento2/wiki/Sources-to-Sales-Channels-Mapping.

AC:

• it should support many-to-many mapping between sources and stocks
• it should provide UI to manage the addition of new stocks and assigning the stock to sources
• it should provide API for the same functionality