Blob reuse decisions do not take into account manifest support

[mtrmac]

E.g. when pushing a schema2 image, dockerImageDestination.TryReusingBlobWithOptions can find and choose to use a Zstd layer

• That directly breaks if the user forced schema1/schema2 manifest type
• Even without copy.Options.ForceManifestMIMEType, if the destination is manifest-schema-agnostic, and determineManifestConversion has no reason to expect Zstd to be involved, the copy code will only have a single candidate; we need to either add an OCI manifest schema candidate, or not reuse Zstd layers if the copy is not set up to use OCI.

[mtrmac]

A reproducer for the first part:

# Create a Zstd-compressed image
skopeo --override-os linux --insecure-policy copy --dest-tls-verify=false --dest-compress-format zstd docker://quay.io/libpod/alpine docker://…/zstd
# The above does not compute/record an uncompressed digest in the BIC, making reuse impossible;
# force decompression to create the uncompressed digest records
skopeo --override-os linux --insecure-policy copy --dest-decompress=true --tls-verify=false docker://quay.io/libpod/alpine dir:t
skopeo --override-os linux --insecure-policy copy --dest-decompress=true --tls-verify=false docker://…/zstd dir:t
# A plain copy now finds the Zstd layer on the destination, and forces a conversion to OCI
skopeo --override-os linux --insecure-policy copy --dest-tls-verify=false docker://quay.io/libpod/alpine docker://…/dest2
# … and a copy forcing manifest schema to non-OCI fails:
skopeo --override-os linux --insecure-policy copy --dest-tls-verify=false --format schema2 docker://quay.io/libpod/alpine docker://…/dest-s2
# … creating an updated image manifest: preparing updated manifest, layer \"sha256:…\": zstd compression is not supported for type \"application/vnd.docker.image.rootfs.diff.tar.gzip\"
# With the fix:
# level=debug msg="Ignoring candidate blob sha256:… in …/dest2, compression zstd does not match required nil or MIME types []string{\"application/vnd.docker.distribution.manifest.v2+json\"}"

The second part is not as easy to reproduce; only dir is manifest-schema-agnostic, but it also does not reuse layers across copies.