Support for JSON Format Output in otdgen

[tmizuma]

Is your feature request related to a problem? Please describe.

The current version of otdgen only supports TypeScript and JavaScript output formats. However, there is a need for a JSON format for use cases such as introducing mock data for server-side applications and native applications.

Describe the solution you'd like

I suggest adding an option to generate output in JSON format by adding the -ext option with the .json file extension.

Describe alternatives you've considered

N/A

Additional context

N/A