cli/container: Pretty-print creation timestamp in list-objects output

Previously, `container list-object` command with `--with-attr` flag
printed objects' attributes in raw way. In particular, creation
timestamp was showed in Unix format (e.g. 1692609422). While this format
is widely known, its human-unreadable. To facilitate the user's
experience, the output should be supplemented with a readable format
similar to the `object head` command.

Use Go `time.Time` stringer and print the result along with Unix time
value. From:
```
HKupeALCqk3VUSmuRXTGdJt6ZZQoJsJ3VtNVV8Pu1Xjd
  Timestamp=1692609422
```
to:
```
HKupeALCqk3VUSmuRXTGdJt6ZZQoJsJ3VtNVV8Pu1Xjd
  Timestamp=1692609422 (2023-08-21 11:17:02 +0200 CEST)
```

Signed-off-by: Leonard Lyubich <[email protected]>

cmd/neofs-cli/modules/container/list_objects.go

@@ -73,7 +73,15 @@ var listContainerObjectsCmd = &cobra.Command{
 				if err == nil {
 					attrs := resHead.Header().UserAttributes()
 					for i := range attrs {
-						cmd.Printf("  %s: %s\n", attrs[i].Key(), attrs[i].Value())
+						key := attrs[i].Key()
+						val := attrs[i].Value()
+
+						if key := attrs[i].Key(); key == object.AttributeTimestamp {
+							cmd.Printf("  %s=%s (%s)\n", key, val, common.PrettyPrintUnixTime(val))
+							continue
+						}
+
+						cmd.Printf("  %s: %s\n", key, val)
 					}
 				} else {
 					cmd.Printf("  failed to read attributes: %v\n", err)