[OCaml] Add key to each model field

[sir4ur0n]

Add a [@key "someField"] annotation to each model field, to avoid mismatches when the JSON field is not in snake_case.

Closes #20765

For steps to reproduce/validate, see the issue

@cgensoul

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[wing328]

thanks for the PR.

can you please PM me via Slack to have a quick discussion on this change in the coming week?

My timezone is +0800 (Hong Kong)

https://join.slack.com/t/openapi-generator/shared_invite/zt-2wmkn4s8g-n19PJ99Y6Vei74WMUIehQA

[sir4ur0n]

I contacted you yesterday on Slack 👋