vendor: add github.com/diegomarangoni/gcscache

also include all transitive dependencies of gcscache, which seems to
bring in the entire world of Google Cloud Platform.

vendor/google.golang.org/api/gensupport/header.go

@@ -0,0 +1,22 @@
+// Copyright 2017 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 gensupport
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+)
+
+// GoogleClientHeader returns the value to use for the x-goog-api-client
+// header, which is used internally by Google.
+func GoogleClientHeader(generatorVersion, clientElement string) string {
+	elts := []string{"gl-go/" + strings.Replace(runtime.Version(), " ", "_", -1)}
+	if clientElement != "" {
+		elts = append(elts, clientElement)
+	}
+	elts = append(elts, fmt.Sprintf("gdcl/%s", generatorVersion))
+	return strings.Join(elts, " ")
+}