review media types

[errordeveloper]

At the moment Tape has config media type application/vnd.docker.tape.config.v1alpha1+json and two-layer media types:

• application/vnd.docker.tape.content.v1alpha1.tar+gzip
• application/vnd.docker.tape.attest.v1alpha1.jsonl+gzip

Is it correct to use the same config media type for both artifacts, perhaps there should be distinct config media types also, so that artifact type can be known before fetching the 2nd level manifest and without having to assume ordering.

[errordeveloper]

This was resolved in #18