One of the features that I really miss is an ability to add custom validation on the schema-level. I would like to use CEL validation language to describe more complex rules for my schema.

Enforcing CEL into ogen is probably too much, so I suggest to add a new x-ogen-validate extension to allow registering custom validators.

Why not use x-oapi-codegen-extra-tags?

I understand from examples that extra tags is one way of doing it.

However, this doesn't allow to register validation on a request struct level, which I think is very useful for writing powerful validations.