Centralize digest validation in public bundle API

pkg/bundle/digest.go

@@ -11,7 +11,8 @@ import (
 
 var digestPattern = regexp.MustCompile(`^sha256:[0-9a-f]{64}$`)
 
-func validateDigest(value string) error {
+// ValidateDigest reports whether value uses the lowercase sha256:<64 hex> form.
+func ValidateDigest(value string) error {
 	if !digestPattern.MatchString(value) {
 		return fmt.Errorf("must be lowercase sha256:<64 hex>")
 	}