Add support for Fulcio and Rekor, and --sign-by-sigstore=param-file

(skopeo copy) and (skopeo sync) now support --sign-by-sigstore=param-file,
using the containers-sigstore-signing-params.yaml(5) file format.

That notably adds support for Fulcio and Rekor signing.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

vendor/google.golang.org/appengine/internal/app_id.go

@@ -0,0 +1,28 @@
+// Copyright 2011 Google Inc. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package internal
+
+import (
+	"strings"
+)
+
+func parseFullAppID(appid string) (partition, domain, displayID string) {
+	if i := strings.Index(appid, "~"); i != -1 {
+		partition, appid = appid[:i], appid[i+1:]
+	}
+	if i := strings.Index(appid, ":"); i != -1 {
+		domain, appid = appid[:i], appid[i+1:]
+	}
+	return partition, domain, appid
+}
+
+// appID returns "appid" or "domain.com:appid".
+func appID(fullAppID string) string {
+	_, dom, dis := parseFullAppID(fullAppID)
+	if dom != "" {
+		return dom + ":" + dis
+	}
+	return dis
+}