Support oneOf without $ref references

[xuorig]

Hey folks, I was trying to diff two large OpenAPI descriptions and I could not get it working due to this exception:

Unexpected exception. Reason: invalid oneOf schema

It's coming from here: https://github.com/OpenAPITools/openapi-diff/blob/master/core/src/main/java/com/qdesrame/openapi/diff/core/compare/schemadiffresult/ComposedSchemaDiffResult.java#L101-L104

I noticed it seems to expect that the schemas under oneOf are references, and not an inline schema? For example:

schema:
  oneOf:
  - type: string
  - type: object
     properties:
       a:
         type: string

Should be valid OpenAPI☝️ But this causes the exception.

[gsdatta]

Seeing the same issue