Support is needed for media type extensions

[mikebrow]

A pattern is desirable for adding media type extensions to OCI media types. This pattern will also be desirable for additional media types being discussed in opencontainers/artifacts.

See issue #747
which resulted in PR #775

Comment from @vbatts from our 3/4/2021 opencontainers call..

@lumjjb ok, finally discussing this in the weekly call.
Since this encrypted layer discussion has begun, the https://github.com/opencontainers/artifacts has come along. To a large degree, this is another mimetype/mediaType that would be documented there, and allowed as an extension to the image-spec.

There is a lot of good work here in the PR, that should transfer to something like a README for these mime-types in the artifacts repo.
Does this make sense?

In the call we came to the agreement that the image-spec should have some basic extension support for mime types and that the details around supported mime extensions should lie in the artifacts repo.

more detail to follow as we get a working group and volunteer(s)