Allow JSON schema id and $schema in OpenAPI schema definitions

JSON Schema files can contain top-level fields id and $schema. When using $ref in an OpenAPI document to reference an external schema, the OpenAPI tooling rejects JSON schema files that contain one or both of these fields because 1) the spec does not allow additional fields and thus 2) the published JSON Schema document for OpenAPI does not allow additional fields. See https://github.com/whitlockjc/swagger-lint/issues/11 for some specific examples.

I propose changing the spec and the corresponding OpenAPI 3.0 schema to allow id and $ref so that one can publish OpenAPI model definition as JSON schema documents. This will allow more tools to consume them more fully.

Note: I am not requesting OpenAPI change the allowed subset of JSON Schema features.

[darrelmiller]

This issue will be addressed if we can make headway on the alternate schema proposal. Whitelisting certain properties I fear will just open the door to whitelisting “just one more property” and the end result will be confusion as to what tooling supports, ignores and doesn’t support.

[philsturgeon]

Added in #1977 so we can close this.