Vendor in c/image with sigstore support

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

vendor/google.golang.org/protobuf/encoding/protojson/doc.go

@@ -0,0 +1,11 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package protojson marshals and unmarshals protocol buffer messages as JSON
+// format. It follows the guide at
+// https://developers.google.com/protocol-buffers/docs/proto3#json.
+//
+// This package produces a different output than the standard "encoding/json"
+// package, which does not operate correctly on protocol buffer messages.
+package protojson