Allow properties on $ref objects for OpenAPI 3.1.

[luhn]

JSON Schema draft 5 (OpenAPI 3.0.4), it specifically says additional properties on a $ref object are ignored.

All other properties in a "$ref" object MUST be ignored.

https://datatracker.ietf.org/doc/html/draft-wright-json-schema-00#section-7

However, draft 2020-12 (OpenAPI 3.1.0) seems to have changed its tune:

Its results are the results of the referenced schema. [[CREF5: Note
that this definition of how the results are determined means that other
keywords can appear alongside of "$ref" in the same schema object. ]]

https://datatracker.ietf.org/doc/html/draft-bhutton-json-schema-00#section-8.2.3.1

So if I'm understanding correctly, things like this are now valid and meaningful:

{
            "$ref": "#/components/schemas/Child",
            "description": "A category",
            "x-extension": "A great extension",
 }