Add CreatedAt format option to podman artifact ls

This change adds a .CreatedAt format option to the podman artifact ls
command to match the behavior of podman images --format CreatedAt.

The .Created field continues to display human-readable elapsed time
(e.g., '6 hours ago'), while the new .CreatedAt field displays the
full timestamp (e.g., '2025-10-23 12:34:56 +0000 UTC').

Changes:
- Refactored artifactListOutput struct to store time.Time value
- Added CreatedAt() method returning full timestamp string
- Added Created() method for human-readable duration
- Updated documentation to include .CreatedAt field
- Added e2e test for .CreatedAt format option

Generated-with: Cursor AI
Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

cmd/podman/artifact/list.go

@@ -38,12 +38,29 @@ type listFlagType struct {
 }
 
 type artifactListOutput struct {
-	Digest      string
-	Repository  string
-	Size        string
-	Tag         string
-	Created     string
-	VirtualSize string
+	Digest       string
+	Repository   string
+	Size         string
+	Tag          string
+	created      time.Time
+	VirtualSize  string
+	virtualBytes int64
+}
+
+// Created returns human-readable elapsed time since artifact was created
+func (a artifactListOutput) Created() string {
+	if a.created.IsZero() {
+		return ""
+	}
+	return units.HumanDuration(time.Since(a.created)) + " ago"
+}
+
+// CreatedAt returns the full timestamp string of when the artifact was created
+func (a artifactListOutput) CreatedAt() string {
+	if a.created.IsZero() {
+		return ""
+	}
+	return a.created.String()
 }
 
 var (
@@ -109,23 +126,23 @@ func outputTemplate(cmd *cobra.Command, lrs []*entities.ArtifactListReport) erro
 			artifactHash = artifactDigest.Encoded()
 		}
 
-		var created string
+		var createdTime time.Time
 		createdAnnotation, ok := lr.Manifest.Annotations[imgspecv1.AnnotationCreated]
 		if ok {
-			createdTime, err := time.Parse(time.RFC3339Nano, createdAnnotation)
+			createdTime, err = time.Parse(time.RFC3339Nano, createdAnnotation)
 			if err != nil {
 				return err
 			}
-			created = units.HumanDuration(time.Since(createdTime)) + " ago"
 		}
 
 		artifacts = append(artifacts, artifactListOutput{
-			Digest:      artifactHash,
-			Repository:  named.Name(),
-			Size:        units.HumanSize(float64(lr.Artifact.TotalSizeBytes())),
-			Tag:         tag,
-			Created:     created,
-			VirtualSize: fmt.Sprintf("%d", lr.Artifact.TotalSizeBytes()),
+			Digest:       artifactHash,
+			Repository:   named.Name(),
+			Size:         units.HumanSize(float64(lr.Artifact.TotalSizeBytes())),
+			Tag:          tag,
+			created:      createdTime,
+			VirtualSize:  fmt.Sprintf("%d", lr.Artifact.TotalSizeBytes()),
+			virtualBytes: lr.Artifact.TotalSizeBytes(),
 		})
 	}