Support for remote/file references

[kylef]

Support for remote (http) and local file references in OAS 2 and 3 schemas along with API Blueprint's Schema keyword.

For example, a schema such as:

{
  "$ref": "./api/attribute.yaml#/attribute"
}

This particular feature needs a lot of thought as it can allow potential security problems for example when you run Dredd against an API Blueprint or OpenAPI document which contains a remote $ref in a schema. The $ref could potentially be used to populate a request body with sensitive data such as /etc/shadow, ~/.npmrc, ~/.ssh/id_rsa etc. This is the particular reason why this has been been supported. JSON Schema can be used to generate request and response bodies.

Additional context

Raised from #1329 (comment)

[kylef]

Duplicate of #676