title: OpenApi 3 import schemas from external files
body: <p>I am defining common schemas for Web services and I want to import them in the components/schema section of the specification.
  I want to create a canonical data model that is common across multiple services to avoid redefining similar objects in each service definition.</p>
  
  <p>Is there a way to do this?
  Is there a similar mechanism to what XSD does with its import tag?</p>