Make the OpenAPINormalizer extensible

[jpfinne]

fix #20994

Allow configuration of the OpenAPINormalizer
New normalizer rule: NORMALIZER_CLASS to provide the class name of the extended OpenAPINormalizer.

Change the private modifiers to protected to allow overriding and extensions of behaviour.

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[wing328]

thanks for the PR

which looks good overall

but what's the reason behind to further breaking down normalizeSchema with 1 additional function doNormalizeSchema?

[jpfinne]

but what's the reason behind to further breaking down normalizeSchema with 1 additional function doNormalizeSchema?

public Schema normalizeSchema(Schema schema, Set<Schema> visitedSchemas) {
has 2 purposes:

1. check if not already processed
2. perform the actual normalization

I want to be able to override the 2.

Another option is to keep the original normalizeSchema and be sure that all options can be overriden.
For example, there is a need to uncomment the handling of normalizeAdditionalProperties

     if (schema.getAdditionalProperties() != null) {
           normalizeAdditionalProperties(m, visitedSchemas);
     }

or split the normalization of composed schema:

} else if (ModelUtils.isComposedSchema(schema)) { // composed schema
   return normalizeComposedSchema(schema, schemaVisited);
}

and copy the content of the inner block to the new protected Schema normalizeComposedSchema() method