lib/container: Expose oci_spec::image::ImageManifest publicly

Previously we had our own internal definitions for these things,
which I didn't want to make public API.

Now that a crate exists for this and we're using it internally,
let's take the next step and expose it as part of our API.

This will allow clients (e.g. rpm-ostree) to render things more nicely.

lib/src/container/unencapsulate.rs

@@ -2,7 +2,7 @@
 //!
 //! This code only operates on container images that were created via
 //! [`encapsulate`].
-//! 
+//!
 //! # External depenendency on container-image-proxy
 //!
 //! This code requires <https://github.com/cgwalters/container-image-proxy>
@@ -190,7 +190,7 @@ pub async fn unencapsulate_from_manifest(
         return Err(anyhow!("containers-policy.json specifies a default of `insecureAcceptAnything`; refusing usage"));
     }
     let options = options.unwrap_or_default();
-    let layer = require_one_layer_blob(&manifest)?;
+    let layer = require_one_layer_blob(manifest)?;
     event!(
         Level::DEBUG,
         "target blob digest:{} size: {}",