Open Api for FileInfo

## 🚀 Feature Requests

When getting a file (e.g. via `getSingleFile`) it'd be nice to also know wether it came from cache or the internet.
We already can do this by manually trying to first get it from cache or otherwise the internet, but the "one function does it all" approach would be nice as well.

Start by tracing the getSingleFile entry point and the FileInfo type to see how cache and internet retrieval are currently represented. Define the source information the API should expose, then add coverage for both cache and internet paths; done means callers can identify which source supplied the file through one function.

索引モデルが issue の本文から書いたものです。