Add AnnotationArtifactCreated constant (#109)

* Add AnnotationArtifactCreated

Signed-off-by: Sylvia Lei <lixlei@microsoft.com>

specs-go/v1/annotation.go

@@ -0,0 +1,20 @@
+// Copyright 2021 ORAS Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//		http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package v1
+
+// AnnotationArtifactCreated is the annotation key representing the UTC date
+// and time on which the artifact was created, in RFC 3339 format.
+// Reference: https://datatracker.ietf.org/doc/html/rfc3339#section-5.6
+const AnnotationArtifactCreated = "io.cncf.oras.artifact.created"