FR: add ocify to convert between OCI and Docker mediaTypes

[thesayyn]

Since this is a manifest manipulation tool, I thought it would be a great addition have a subcommand called ocify or convert to convert images between docker v2 and OCI mediaTypes and Config.

An example cmd snippet

manifest-tool convert localhost:8080/image:latest  localhost:8080/image-oci:latest --format="oci|docker"

I'd be happy to implement this if that makes sense.