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/meta/parse.go

@@ -289,6 +289,7 @@ func SetImageMetaFromInput(ctx context.Context, repo, reference, mediaType strin
 				mTypes.SignatureMetadata{
 					SignatureType:   sigType,
 					SignatureDigest: digest.String(),
+					SignatureTag:    reference,
 					LayersInfo:      layers,
 				})
 			if err != nil {
@@ -342,6 +343,11 @@ func isSignature(reference string, manifestContent ispec.Manifest) (bool, string
 		return true, NotationType, manifestContent.Subject.Digest
 	}
 
+	// check cosign signature
+	if manifestArtifactType == zcommon.ArtifactTypeCosign && manifestContent.Subject != nil {
+		return true, CosignType, manifestContent.Subject.Digest
+	}
+
 	if tag := reference; zcommon.IsCosignTag(reference) {
 		prefixLen := len("sha256-")
 		digestLen := 64