fix(customer): Unique constraint on customer email (#7439)

**What**
Prevent creating multiple customers with the same email

packages/modules/customer/mikro-orm.config.dev.ts

@@ -1,8 +1,12 @@
 import * as entities from "./src/models"
+import { TSMigrationGenerator } from "@medusajs/utils"
 
 module.exports = {
   entities: Object.values(entities),
   schema: "public",
   clientUrl: "postgres://postgres@localhost/medusa-customer",
   type: "postgresql",
+  migrations: {
+    generator: TSMigrationGenerator,
+  },
 }