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.