OpenApi 3.0 support to be extensible

The current implementation has an extension mechanism where users can inject AsciiDoc/Markdown at certain positions. Ideally, OpenApi 3.0 support #340 should be allow extension too.

@austek I believe a way already been provided under #393 . For any further component that we create we should make use of it like done in OverviewDocument for OpenAPI 3.

In that case just providing test for a sample extension should be sufficient