start using GetRawBlob

Let's start using https://docs.rs/containers-image-proxy/latest/containers_image_proxy/struct.ImageProxy.html#method.get_raw_blob

Hmm we may want to add a method to that crate which detects when the proxy has a new enough semver so we can do dynamic dispatch.

That will fix two big things:

- We can easily actually switch to fetching layers in parallel
- Avoid the giant mess of FinishBlob and decompressors (ref https://github.com/bootc-dev/bootc/pull/1415 )