Get & list cached files

## 🚀 Feature Requests

The default `CacheManager` is great, and automatic caching is great.
But for some files, I need to manually download them to cache, using `downloadFile`, great.
But later, I need to check wether a file is already cached, to decide whether to download it or not.
And I need to do that for hundreds of files.

So it would be very nice to add too methods :
1. A method to check whether a file is cached or not (without providing the File itself), returning a bool.
2. A method to list ALL cached files (like `provider.getAllObjects()` in `CacheStore`), returning a list of keys.

Start by reading CacheManager.downloadFile and the CacheStore provider.getAllObjects() path to understand how cache keys and stored files are represented. Trace the existing cache lookup flow before deciding where the two APIs belong. Done means callers can check a key without a File and retrieve all cached keys, including the requested behavior for hundreds of files.

Vom Indexierungsmodell aus dem Issue-Text verfasst.