Issue Description:

It seems that working with an image that have a layer with media type application/vnd.oci.image.layer.v1.tar+zstd is not possible. When I try to encrypt or even use layerinfo I see that the following error is returned:

ctr: unhandled media type application/vnd.oci.image.layer.v1.tar+zstd: invalid argument

Also $ ctr-enc containers create repo:port/zstd-image:latest some-id fails with:

ctr: you are not authorized to use this image: bad/unhandled MediaType application/vnd.oci.image.layer.v1.tar+zstd in encryptChildren

The zstd compression format is supported by containerd and for example $ ctr containers create repo:port/zstd-image:latest hello-world-zstd is working as expected and the desired container is created successfully.

Steps to reproduce:

To reproduce the issue one can use skopeo to change the compression of a simple hello-world image and then push it to a private/local registry, e.g.:

1. Use skopeo copy to create a zstd-compressed image and push it locally:

$ skopeo copy --dest-compress-format=zstd docker://hello-world docker://<some-plain-http-local-ip:port>/hello-world-zstd:latest --dest-tls-verify=false

2. Use skopeo inspect to check the result pushed image compression:

$ skopeo inspect --raw docker://<some-plain-http-local-ip:port>/hello-world-zstd:latest --tls-verify=false | jq -r '.layers[].mediaType'

3. Pull the image:

$ ctr-enc image pull --plain-http <some-plain-http-local-ip:port>/hello-world-zstd:latest

4. Try any of the following commands:

$ ctr-enc image encrypt <some-plain-http-local-ip:port>/hello-world-zstd:latest --recipient=pgp:[email protected]

$ ctr-enc containers create <some-plain-http-local-ip:port>/zstd-image:latest some-id

Proposal:

In my opinion the compression part of the media type should not be taken into account as (please correct me if I am wrong) it is not relevant to the encryption process. So, the checks that are done here, here and probably in some other parts of the code that I haven't explore, could be refactored in a way that disregards the compression type, whether it is zstd or something else.
As a proof of my thoughts I have done locally the most simple test, which is just adding application/vnd.oci.image.layer.v1.tar+zstd and application/vnd.oci.image.layer.v1.tar+zstd+encrypted to the switch/case blocks in the functions linked above and as expected the encryption and creation of the container was successful. So, I think that it would be nice to have more flexibility regarding the verification of media types that are supported by the library.