Support OpenAPI3

[stuart williams]

ASP.NET 3.1 LTS and higher produce OpenAPI3 as in this snippet:

{
  "openapi": "3.0.1",
  ...
}

Produces this error

FATAL: Error: File 'file:///....json' is not a valid OpenAPI 2.0 definition (expected 'swagger: 2.0')

It would be good to support OpenAPI3. See spec: https://swagger.io/specification/

[Adrien CABARBAYE]

Similar issue #148

[stuart williams]

try https://github.com/Tufin/oasdiff