start using GetRawBlob

[cgwalters]

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 unencapsulate: use "inner" stream when finishing Decompressor #1415 )