Extract format_string function.

Extract format_string function and add a test.

Test: run linker-unit-tests
Change-Id: I794a29aaf62e184438ce1a9224b88aa0586c17b5
diff --git a/linker/linker_utils.h b/linker/linker_utils.h
index 5881688..740d04b 100644
--- a/linker/linker_utils.h
+++ b/linker/linker_utils.h
@@ -34,6 +34,8 @@
 
 extern const char* const kZipFileSeparator;
 
+void format_string(std::string* str, const std::vector<std::pair<std::string, std::string>>& params);
+
 bool file_is_in_dir(const std::string& file, const std::string& dir);
 bool file_is_under_dir(const std::string& file, const std::string& dir);
 bool normalize_path(const char* path, std::string* normalized_path);