Ignore vendor extensions in new renderer

[ikalnytskyi]

The new renderer does not assume that there may be vendor extensions in OAS objects. I'm not sure what's going to happen, I can assume the renderer will either fail or render garbage when vendor extensions are met. We need to ensure they are ignored.

For this, I think we should implement some helper function, e.g. oas_iteritems(vendor_extensions=False) that can iterate via OAS objects with or without vendor extensions based on the parameter, and use that helper inside the renderer.