Config for Extensions Manifest Directory

The directory for Firebase **extensions** manifest is currently hardcoded as `"extensions"`. This restricts flexibility for users who want to organize their project structure differently, especially in monorepos or more complex setups where Firebase configs are grouped in specific directories.

In many cases, it would be beneficial to have the ability to specify the directory for Firebase extensions in the firebase.json configuration file, just like how we can specify paths for Firestore rules, indexes, storage rules, and other Firebase-related configurations.

### Code Reference
https://github.com/firebase/firebase-tools/blob/3e84ee86e98eb51df7ebb325a4b9a0572bb031df/src/extensions/manifest.ts#L16