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/commands/plugin/db/generate.ts

@@ -1,11 +1,12 @@
-import { glob } from "glob"
 import { logger } from "@medusajs/framework/logger"
 import {
-  toUnixSlash,
+  defineMikroOrmCliConfig,
   DmlEntity,
   dynamicImport,
-  defineMikroOrmCliConfig,
+  isFileSkipped,
+  toUnixSlash,
 } from "@medusajs/framework/utils"
+import { glob } from "glob"
 import { dirname, join } from "path"
 
 import { MetadataStorage } from "@mikro-orm/core"
@@ -68,6 +69,9 @@ async function getEntitiesForModule(path: string) {
 
   for (const entityPath of entityPaths) {
     const entityExports = await dynamicImport(entityPath)
+    if (isFileSkipped(entityExports)) {
+      continue
+    }
 
     const validEntities = Object.values(entityExports).filter(
       (potentialEntity) => {