Add Code Generator for idl_gen_fbs to parse .proto files (#7832)

* Add code generator for proto files

* Update

* Add --proto to script

* Remove cmt

* Move proto parsing logic into else block to share same set up logic for code_generator

* Remove IsValidCodeGenerator

include/flatbuffers/idl.h

@@ -717,6 +717,7 @@ struct IDLOptions {
     kKotlin = 1 << 15,
     kSwift = 1 << 16,
     kNim = 1 << 17,
+    kProto = 1 << 18,
     kMAX
   };