Custom JSONEncoder & JSONDecoder

This MR adds custom JSONEncoder and JSONDecoder that handles non-standard JSON values used by the go client.

• Handle BigInt values in JSON at key Value by converting them to/from String.
• Add JSONEncodingError and JSONDecodingError for extra verbose output when debugging JSON coding-related issues.

The JSONEncoder and JSONDecoder are internal, to avoid name collisions when using the XXClient library as a dependency, but internally all models are encoded/decoded using the custom encoder/decoder. Convenient encode and decode methods on each model remain public.