CLI option to validate-only the schema

[adrianlungu]

Description

Tried redocly and vacuum for validating an OpenAPI schema, but there are still differences with regards to what ogen checks vs what the other tools test so it would be nice to have a validate-only cli option to be able to use it in CI to check if the schema is valid from ogen's perspective.

For example:

        - name: status
          description: The status
          example: active
          schema:
            type: string
            items:
              type: string
              enum:
                - active
                - inactive

This throws a items: unexpected field for type "string" via ogen but redocly and vacuum consider this a valid OpenAPI spec for some reason.