Add generation of JSON Schema to library (#6165)

* 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

include/flatbuffers/idl.h

@@ -1008,6 +1008,10 @@ extern bool GenerateText(const Parser &parser, const void *flatbuffer,
 extern bool GenerateTextFile(const Parser &parser, const std::string &path,
                              const std::string &file_name);
 
+// Generate Json schema to string
+// See idl_gen_json_schema.cpp.
+extern bool GenerateJsonSchema(const Parser &parser, std::string *json);
+
 // Generate binary files from a given FlatBuffer, and a given Parser
 // object that has been populated with the corresponding schema.
 // See code_generators.cpp.