Introduce SourceCode attribute for Source and SourceItem entities

Currently our Source has just next fields:

• id
• name
• description ….

SourceItem Import/Export returns references to Sources by Source_ID but in Magento Source ID is auto-generated surrogate key and if we import data from external ERP system - we need to use something not so specific to Magento internals. Source Name is not a good choice for that because even now, we use "Default Source" and "Default Stock" for our default entities. That doesn’t look like identifier, but just something meaningful to read. From this perspective - adding additional unique field " Source Code" could be an option for us. Moreover, in Product Import story @globalmark specified https://github.com/magento/inventory/wiki/Update-Product-import-export-to-include-inventory-source

default=123|source2=150 as a possible field for Qty

What are default and source2 in this example? Also, we need to make our Product Import being compatible with Source Inventory import. So that, if we use Source Code in one, we need to use the same identity in another one as well

Story Tickets

Filter for all the story tickets has label: SourceCode