Support for external references in $ref

Created by: jmini

According to the specification (Reference Object in OAS3) it is allowed to define references to external files.

For example looking for the Pet schema in other.yaml in the same repository:

$ref: ‘./other.yaml#/components/schemas/Pet'

It looks like that the other input file does not need to be a “OpenAPI” yaml/json file at all, it can just be a sub-part of a part:

$ref: definitions.yaml#/Pet

This is not supported at all by OpenAPI generator.