Unknown Artifact Icon

[SteveLasker]

As artifact authors submit their icons for their types, we'd like to have a default one for unknown.
Thinking it would be some variation of what we do for the artifacts logo/icon.
The quick story is people are using application/vnd.oci.image.config.v1+json as the default, which I believe should be reserved for images that can be run with docker run and other true image tools.
We'd like a default icon for the unknown types, which should be discouraged, but without having a default unknown type, people will likely use oci.image which will fail when using container tools.
See https://github.com/SteveLasker/RegistryArtifactTypes/blob/master/mediaTypes.md#media-types for an example of what a listing could look like.