[REQ] Make OpenApiNormalizer extensible

[jpfinne]

Is your feature request related to a problem? Please describe.

The current OpenApiNormalizer can not be extended. It is initialized in DefaultGenerator with a new OpenAPINormalizer(). So there is not way to override or extends the current implementation.

Customization is only available at the level of libraries or mustache templates.

Describe the solution you'd like

Add a configurable factory constructor to the default OpenapiNormalizer.
Change the private methods to protected methods.

Describe alternatives you've considered

Add the desired customization in the standard code base.

Additional context