Add String Formatting Util

(Merged from http://go/wvgerrit/160042.)

Since we don't have access to std::format yet, this patch adds a
function to wvutil to format text into a std::string.

Bug: 255466913
Test: x86-64
Test: raven
Change-Id: I28043da76af5b4772a29fa7e7241343caf9b54a1

libwvdrmengine/cdm/util/include/string_format.h

@@ -0,0 +1,18 @@
+// Copyright 2022 Google LLC. All Rights Reserved. This file and proprietary
+// source code may only be used and distributed under the Widevine License
+// Agreement.
+#ifndef WVCDM_UTIL_STRING_FORMAT_H_
+#define WVCDM_UTIL_STRING_FORMAT_H_
+
+#include <string>
+
+namespace wvutil {
+
+#ifdef __GNUC__
+[[gnu::format(printf, 2, 3)]]
+#endif
+bool FormatString(std::string* out, const char* fmt, ...);
+
+}  // namespace wvutil
+
+#endif  // WVCDM_UTIL_STRING_FORMAT_H_