feat(medusa): Create default sales channel associated to the store (#1830)

**What**
Add support for default sales channel

**How**
- Implement a new method in the salesChannelService `createDefault`
- call the new method above in the default loader

**Test**
- Unit tests of the sales channel service method createDefaulta
- Init default loader unit tests

Fixes CORE-316

packages/medusa/src/models/sales-channel.ts

@@ -2,7 +2,6 @@ import { BeforeInsert, Column } from "typeorm"
 
 import { SoftDeletableEntity } from "../interfaces"
 import { FeatureFlagEntity } from "../utils/feature-flag-decorators"
-import { resolveDbType } from "../utils/db-aware-column"
 import { generateEntityId } from "../utils"
 
 @FeatureFlagEntity("sales_channels")