[Feature Request] Make all model class can be ser/deserializable in JSON format.

[kazuki-ma]

Feature Request

Describe the solution you'd like

• Save message object by some serialization form is convenient way to create proof of concept.
• And some developers and 3rd parties create admin tool for official account, they need store messages for reservation delivery. If model class can be deserialize from JSON form, it's suitable.

Describe alternatives you've considered

If we don't support it, each developers should implement their original serialization format and implement ser/de. It's ok but alternative out of box way may be welcomed for proof of concept.

Additional context

[kazuki-ma]

https://github.com/line/line-bot-sdk-java/pull/254

[tokuhirom]

I guess the requirements are solved by current implementation. But if there's any problems, any users feel free to send PR or just generate other client library based on openapi spec.