Add extension points for custom OpenApi\Generator instantiation

[DerManoMann]

Related to #667

Summary

• Add Generator::newOpenApiGenerator() method to isolate OpenApi\Generator creation
• Add GeneratorFactory::createGenerator() method to allow custom Generator instantiation
• These two methods together allow customizing the OpenApi\Generator instance without
  duplicating the entire Generator or GeneratorFactory class

Motivation

Projects that need a pre-configured OpenApi\Generator (e.g. via OpenApiBuilder from
openapi-extras) currently have to override both GeneratorFactory and Generator entirely.
With these extension points, a single custom factory suffices:

class CustomGeneratorFactory extends GeneratorFactory
{
    protected function createGenerator(
        array $paths,
        array $constants,
        bool $yamlCopyRequired,
        SecurityDefinitions $security,
        array $scanOptions
    ): Generator {
        return new class($paths, $constants, $yamlCopyRequired, $security, $scanOptions) extends Generator
            protected function newOpenApiGenerator(): OpenApiGenerator
            {
                return new OpenApiGenerator();
            }
        };
    }
}

[coveralls]

coverage: 99.396% (+0.02%) from 99.381% — DerManoMann:generator-factory-method into DarkaOnLine:master

[DarkaOnLine]

Closing this one as we did #668 and #670