Added support for nested flatbuffers serializing to json (#4391)

* added bool for nested_flatbuffer
setting bool flag nested_flatbuffer according to attributes
setting nested type

* added JSON serialization for nested flatbuffers

* simplified lookup of nested_flatbuffer StructDef

* added nested_flatbuffer parsing workound relying on flexbuffers

* moved nested flatbuffer parsing into its own function for clarity
removed flexbuffers workound to simplify and speed up nested_flatbuffer parsing
added support for 'legacy' nested_flatbuffers, ubyte arrays

* inlined functions for nested parser init/clean since only used once

* whitespace formatting

* changed type of FieldDef.nested_flatbuffer from bool to StructDef*
removed subsequent type lookups

* removed copies of unrequired data when initializing nested parser

* applied changes requested by reviewer

* removed superfluous lookup of nested_flatbuffer field attributes

* renamed camelCased variables to snake_case