Expose blob `has`/`status` functions

Please expose the following functions:
- `blobs::Client::status`
- `blobs::Client::has`

I was looking at the JS API and I believe it's missing them. Not sure if the C FFI has them or not.

It's important for clients to know what blobs are already downloaded and which are not, to avoid triggering extra unnecessary downloads, or for gossip-based broadcasting of who has what blob on request.