We were occasionally trying to compute schema 2 version 1 signatures on the *unicode* representation, which was failing the signature check. This PR adds a new wrapper type called `Bytes`, which all manifests must take in, and which handles the unicodes vs encoded utf-8 stuff in a central location. This PR also adds a test for the manifest that was breaking in production.
This change ensures that if a manifest list is requested with an accepts header for a *schema 2* manifest, the legacy manifest (if any) is returned as schema 2 if it was pushed as a schema 2 manifest (rather than being auto-converted to schema 1)
This adds additional required properties and methods to the Docker schema interface to allow us to treat both schema1 and schema2 manifests and lists logically equivalent from the OCI mode perspective
