improvement(validation): adopt @webron's OpenAPI 3.0 schema (via #1984)

* adopt @webron's OpenAPI 3.0 schema from OAI/OpenAPI-Specification#1270

permalink: https://github.com/OAI/OpenAPI-Specification/blob/92e15eba1d4591ebfe8c11898c48241e72854381/schemas/v3.0/schema.yaml

* use literal `$ref` key in Reference Object schema

* `"$ref"` -> `$ref`

Author: shockey