V2: Automatically create a webhook parser

[Yang-33]

When a new webhook type is added, it must be updated manually.

If we can prevent human errors, such as forgetting to update, that would be wonderful.

I believe this can be achieved in several ways, each being a realistic option:

1. Write it with an OpenAPI generator
   • However, since the OpenAPI generator is not well-suited for this, it might require significant workarounds.
2. Scan the generated code (https://github.com/line/line-bot-sdk-ruby/tree/38d89d155ed80f4ad12401be72da790bf000ee53/lib/line/bot/v2/webhook/model), create a complete mapping, and then generate the parser.
3. Scan the webhook.yaml and implement it.
4. Like our line-bot-sdk-python, create a parse function within the class that uses polymorphism.

[Yang-33]

How line-bot-sdk-php resolved this: line/line-bot-sdk-php#677