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/identity_flex.go

@@ -0,0 +1,11 @@
+// Copyright 2018 Google LLC. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+// +build appenginevm
+
+package internal
+
+func init() {
+	appengineFlex = true
+}