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/test/coverage-test.mk

@@ -41,6 +41,7 @@ LOCAL_SRC_FILES := \
   ../util/test/cdm_random_unittest.cpp \
   ../util/test/file_store_unittest.cpp \
   ../util/test/file_utils_unittest.cpp \
+  ../util/test/string_format_unittest.cpp \
   ../util/test/test_sleep.cpp \
   ../../oemcrypto/test/oec_device_features.cpp \
   ../../oemcrypto/test/oec_key_deriver.cpp \