Add Index support for artifact type

[SteveLasker]

Tracks adding a config descriptor to index, enabling index to be a type of artifact. This would support:

• Notary v2 signatures, persisted as indexes, enabling existing linkage and garbage collection
• CNAB

[SteveLasker]

Adding a link @radu-matei added a while back: https://github.com/radu-matei/image-spec/pull/1

[SteveLasker]

Closing as we don't see Index being capable of supporting the scenarios captured in #27, #29 and #37