**What**
- added `SalesChannel` entity
- added `SalesChannel` repository
- added `SalesChannel` relations to the order, cart and store entities
- added a migrations file
**How**
- introduced entities and relations under a new feature flag "sales-channels"
Fixes CORE-271
Co-authored-by: Philip Korsholm <88927411+pKorsholm@users.noreply.github.com>