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>

src/flatc.cpp

@@ -250,6 +250,8 @@ const static FlatCOption flatc_options[] = {
   { "", "no-leak-private-annotation", "",
     "Prevents multiple type of annotations within a Fbs SCHEMA file. "
     "Currently this is required to generate private types in Rust" },
+  { "", "file-names-only", "",
+    "Print out generated file names without writing to the files"},
 };
 
 auto cmp = [](FlatCOption a, FlatCOption b) { return a.long_opt < b.long_opt; };
@@ -653,6 +655,9 @@ FlatCOptions FlatCompiler::ParseFromCommandLineArguments(int argc,
       } else if (arg == "--annotate") {
         if (++argi >= argc) Error("missing path following: " + arg, true);
         options.annotate_schema = flatbuffers::PosixPath(argv[argi]);
+      } else if(arg == "--file-names-only") {
+        // TODO (khhn): Provide 2 implementation
+        options.file_names_only = true;
       } else {
         if (arg == "--proto") { opts.proto_mode = true; }