Add flatc option to inhibit all warnings #6005 (#6301)

* Fix C/C++ Create<Type>Direct with sorted vectors

If a struct has a key the vector has to be sorted. To sort the vector
you can't use "const".

* Changes due to code review

* Improve code readability

* Add generate of JSON schema to string to lib

* option indent_step is supported

* Remove unused variables

* Fix break in test

* Fix style to be consistent with rest of the code

* Add option --no-warnings to inhibit all warnings

* Fix order of member initialization

* Add documentation for --no-warnings

include/flatbuffers/idl.h

@@ -572,6 +572,7 @@ struct IDLOptions {
   std::string proto_namespace_suffix;
   std::string filename_suffix;
   std::string filename_extension;
+  bool no_warnings;
 
   // Possible options for the more general generator below.
   enum Language {
@@ -662,6 +663,7 @@ struct IDLOptions {
         cs_gen_json_serializer(false),
         filename_suffix("_generated"),
         filename_extension(),
+        no_warnings(false),
         lang(IDLOptions::kJava),
         mini_reflect(IDLOptions::kNone),
         require_explicit_ids(false),