Replace flatbuffers with json serialization library

[coder137]

https://github.com/nlohmann/json

Why?

• Has easier serialization APIs for C++ STL storage
• Can add custom types (C++ structs -> json)
• Can output in human readable (json) and binary format (ubjson, cbor, messagepack etc)

Problems with Flatbuffers

• Harder to create a C++ to FBS interface
• For JSON like output need to use flexbuffers instead of flatbuffers (no array like root_type allowed)
• Schema -> C++ conversion is harder, since there is no explicit support for hashmaps/hashtables/maps/tables.

TODO

• Add nlohmann::json library as a submodule
• Migrate custom_generator.fbs
• Custom Generator Unit tests
• Migrate target.fbs
• Target Unit tests
• Migrate FileGenerator and TemplateGenerator from flexbuffer to json
• Remove flatbuffers dependency and delete third_party flatbuffers submodule
• Update documentation for flatbuffers -> json migration

[coder137]

#223 Initial JSON Serialization library + CustomGenerator addition
#224 Custom Generator unit tests

[coder137]

#225 JSON Serialization for Target

[coder137]

#226 Remove flatbuffer/flexbuffer usage and library from BuildCC