[PHP] Add support for Generation Gap pattern

Description

Generation Gap pattern is useful for extending codes which generated.

Generation Gap pattern

Generation Gap martinfowler.com
https://martinfowler.com/dslCatalog/generationGap.html

Generation Gap is about keeping the generated and handwritten parts separate by putting them in different classes linked by inheritance.

Sample

class HandwrittenPet extends Pet
{
    // just a sample.
    // it has room for improvement.
    public function canNotBuy()
    {
        return $this->status === ‘pending’ || $this->status === ‘sold’;
    }
}

$api = new PetApi();

// I'd like to add `setReturnType()` to specify the ReturnType.
$api->setReturnType(‘\Path\To\HanwrittenPet’);

$pet = $api->getPetById(1);
var_dump($pet);
// class \Path\To\HanwrittenPet

var_dump($pet->canNotBuy());
// bool

Swagger-codegen version

2.3.0+

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How you do you feel about it?

No files, tests, or entry points are named in the issue. Start by locating the PHP generator/template responsible for the sample PetApi output and define how generated and handwritten classes should be separated; done means the requested return-type behavior is specified and covered for generated PHP clients.