feat(medusa): Implement the SC migration scripts (#2037)

**What**
Migrate the existing products to the default sales channel

FIXES CORE-434

packages/medusa/src/scripts/sales-channels-migration.ts

@@ -0,0 +1,86 @@
+import dotenv from "dotenv"
+import { createConnection } from "typeorm"
+import Logger from "../loaders/logger"
+import { Product } from "../models/product";
+import { Store } from "../models/store";
+
+dotenv.config()
+
+const typeormConfig = {
+  type: process.env.TYPEORM_CONNECTION,
+  url: process.env.TYPEORM_URL,
+  username: process.env.TYPEORM_USERNAME,
+  password: process.env.TYPEORM_PASSWORD,
+  database: process.env.TYPEORM_DATABASE,
+  migrations: [process.env.TYPEORM_MIGRATIONS as string],
+  entities: [process.env.TYPEORM_ENTITIES],
+  logging: true,
+}
+
+const migrate = async function ({ typeormConfig }): Promise<void> {
+  const connection = await createConnection(typeormConfig)
+
+  const BATCH_SIZE = 1000
+
+  await connection.transaction(async (manager) => {
+    const store = await manager
+      .createQueryBuilder()
+      .from(Store, "store")
+      .select("store.default_sales_channel_id")
+      .getOne() as Store
+
+    if (!store.default_sales_channel_id) {
+      Logger.error(`The default sales channel does not exists yet. Run your project and then re run the migration.`)
+      process.exit(1)
+      return
+    }
+
+    let shouldContinue = true
+    while (shouldContinue) {
+      const products = await manager
+        .createQueryBuilder()
+        .from(Product, "product")
+        .leftJoin("product_sales_channel", "product_sc", "product_sc.product_id = product.id")
+        .andWhere("product_sc.product_id IS NULL")
+        .select("product.id as id")
+        .distinct(true)
+        .limit(BATCH_SIZE)
+        .getRawMany()
+
+      if (products.length > 0) {
+        await manager
+          .createQueryBuilder()
+          .insert()
+          .into("product_sales_channel")
+          .values(
+            products.map((product) => ({
+              sales_channel_id: store.default_sales_channel_id,
+              product_id: product.id
+            }))
+          )
+          .orIgnore()
+          .execute()
+      }
+
+      const danglingProductCount = await manager
+        .createQueryBuilder()
+        .from(Product, "product")
+        .leftJoin("product_sales_channel", "product_sc", "product_sc.product_id = product.id")
+        .andWhere("product_sc.product_id IS NULL")
+        .getCount()
+      shouldContinue = !!danglingProductCount
+    }
+  })
+
+  Logger.info(`Product entities have been successfully migrated`)
+  process.exit()
+}
+
+migrate({ typeormConfig })
+  .then(() => {
+    Logger.info("Database migration completed successfully")
+    process.exit()
+  })
+  .catch((err) => console.log(err))
+
+export default migrate