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/github.com/googleapis/gax-go/header.go

@@ -0,0 +1,24 @@
+package gax
+
+import "bytes"
+
+// XGoogHeader is for use by the Google Cloud Libraries only.
+//
+// XGoogHeader formats key-value pairs.
+// The resulting string is suitable for x-goog-api-client header.
+func XGoogHeader(keyval ...string) string {
+	if len(keyval) == 0 {
+		return ""
+	}
+	if len(keyval)%2 != 0 {
+		panic("gax.Header: odd argument count")
+	}
+	var buf bytes.Buffer
+	for i := 0; i < len(keyval); i += 2 {
+		buf.WriteByte(' ')
+		buf.WriteString(keyval[i])
+		buf.WriteByte('/')
+		buf.WriteString(keyval[i+1])
+	}
+	return buf.String()[1:]
+}