feat(utils): define file config (#13283)

** What
 - Allow auto-loaded Medusa files to export a config object.
 - Currently supports isDisabled to control loading.
 - new instance `FeatureFlag` exported by `@medusajs/framework/utils`
 - `feature-flags` is now a supported folder for medusa projects, modules, providers and plugins. They will be loaded and added to `FeatureFlag`

** Why
 - Enables conditional loading of routes, migrations, jobs, subscribers, workflows, and other files based on feature flags.

```ts
// /src/feature-flags

import { FlagSettings } from "@medusajs/framework/feature-flags"

const CustomFeatureFlag: FlagSettings = {
  key: "custom_feature",
  default_val: false,
  env_key: "FF_MY_CUSTOM_FEATURE",
  description: "Enable xyz",
}

export default CustomFeatureFlag
```

```ts
// /src/modules/my-custom-module/migration/Migration20250822135845.ts

import { FeatureFlag } from "@medusajs/framework/utils"

export class Migration20250822135845 extends Migration {
  override async up(){ }
  override async down(){ }
}

defineFileConfig({
  isDisabled: () => !FeatureFlag.isFeatureEnabled("custom_feature")
})
```

integration-tests/modules/__tests__/price-lists/admin/price-lists.spec.ts

@@ -11,7 +11,7 @@ import { createVariantPriceSet } from "../../../helpers/create-variant-price-set
 
 jest.setTimeout(50000)
 
-const env = { MEDUSA_FF_MEDUSA_V2: true }
+const env = {}
 const adminHeaders = {
   headers: { "x-medusa-access-token": "test_token" },
 }

integration-tests/modules/__tests__/price-lists/store/get-product.ts

@@ -1,5 +1,5 @@
-import { PriceListStatus, PriceListType } from "@medusajs/utils"
 import { medusaIntegrationTestRunner } from "@medusajs/test-utils"
+import { PriceListStatus, PriceListType } from "@medusajs/utils"
 import {
   createAdminUser,
   generatePublishableKey,
@@ -15,9 +15,7 @@ const adminHeaders = {
   },
 }
 
-const env = {
-  MEDUSA_FF_MEDUSA_V2: true,
-}
+const env = {}
 
 medusaIntegrationTestRunner({
   env,