feat: add support for oci1.1 cosign signatures(using referrers) (#1963)

- Cosign supports 2 types of signature formats:

	1. Using tag -> each new signature of the same manifest is
	added as a new layer of the signature manifest having that
	specific tag("{alghoritm}-{digest_of_signed_manifest}.sig")

	2. Using referrers -> each new signature of the same manifest is
	added as a new manifest

- For adding these cosign signature to metadb, we reserved index 0 of the
list of cosign signatures for tag-based signatures. When a new tag-based
signature is added for the same manifest, the element on first position
in its list of cosign signatures(in metadb) will be updated/overwritten.
When a new cosign signature(using referrers) will be added for the same
manifest this new signature will be appended to the list of cosign
signatures.

Signed-off-by: Andreea-Lupu <andreealupu1470@yahoo.com>

pkg/storage/common/common.go

@@ -613,6 +613,11 @@ func IsSignature(descriptor ispec.Descriptor) bool {
 			return true
 		}
 
+		// is cosign signature (OCI 1.1 support)
+		if descriptor.ArtifactType == zcommon.ArtifactTypeCosign {
+			return true
+		}
+
 		// is notation signature
 		if descriptor.ArtifactType == zcommon.ArtifactTypeNotation {
 			return true