codegen: OpenAPI export

[mr-c]

Goal: produce an OpenAPI compatible Schema Object that uses as many of the OpenAPI concepts as possible to enable OpenAPI based libraries to validate inline CWL objects as part of other specification

Motivation: opengeospatial/ogcapi-processes#282 (comment)

[An OpenAPI Schema Object definition for CWL] would greatly help promote the use of CWL as a fully-defined OGC Application Package and would help validate schema received by the API.