Add a FileWriter interface (#7821)

* Add a FileWriter interface

* Change interface

* Provide 2 impl for File interface: FileManager & FileNameManager

* Update

* update

* Update

* Add file_writer file

* Update

* Format files

* Update based on review

* Update

* Format bzl file

* Add LoadFile function

* Format

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>

include/flatbuffers/file_manager.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2023 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef FLATBUFFERS_FILE_MANAGER_H_
+#define FLATBUFFERS_FILE_MANAGER_H_
+
+#include <set>
+#include <string>
+
+#include "flatbuffers/util.h"
+
+namespace flatbuffers {
+
+// A File interface to write data to file by default or
+// save only file names
+class FileManager {
+ public:
+  FileManager() = default;
+  virtual ~FileManager() = default;
+
+  virtual bool SaveFile(const std::string &absolute_file_name,
+                        const std::string &content) = 0;
+
+  virtual bool LoadFile(const std::string &absolute_file_name,
+                        std::string *buf) = 0;
+
+ private:
+  // Copying is not supported.
+  FileManager(const FileManager &) = delete;
+  FileManager &operator=(const FileManager &) = delete;
+};
+
+}  // namespace flatbuffers
+
+#endif  // FLATBUFFERS_FILE_MANAGER_H_