feat: provide json schema for ogen config

[ernado]

So we can add support for it into IDE-s.

[utherbit]

Hello, I made an example configuration file, but only for yml. the json configuration would be displayed in a similar way.

See here https://github.com/utherbit/ogen/blob/main/examples/_config/example_all.yml

After the last update it was hard to figure it out without documentation and examples. So I would like this file to be added to the repository, so that it would be easier for users to understand

[ernado]

Thank you!