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")
})
```

packages/medusa/src/api/admin/feature-flags/route.ts

@@ -1,7 +1,4 @@
-import {
-  MedusaRequest,
-  MedusaResponse,
-} from "@medusajs/framework/http"
+import { MedusaRequest, MedusaResponse } from "@medusajs/framework/http"
 import { ContainerRegistrationKeys } from "@medusajs/framework/utils"
 
 export const AUTHENTICATE = false
@@ -13,14 +10,14 @@ export const GET = async (
   const featureFlagRouter = req.scope.resolve(
     ContainerRegistrationKeys.FEATURE_FLAG_ROUTER
   ) as any
-  
+
   const flags = featureFlagRouter.listFlags()
-  
+
   // Convert array of flags to a simple key-value object
   const featureFlags: Record<string, boolean> = {}
-  flags.forEach(flag => {
+  flags.forEach((flag) => {
     featureFlags[flag.key] = flag.value
   })
-  
+
   res.json({ feature_flags: featureFlags })
-}
+}