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/main.go

@@ -0,0 +1,16 @@
+// 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.
+
+// +build appengine
+
+package internal
+
+import (
+	"appengine_internal"
+)
+
+func Main() {
+	MainPath = ""
+	appengine_internal.Main()
+}