[php-slim4] Add Mock Server

Created by: ybelenko

Added Mock Server as fully pre-configured dev only feature.

UPD: how it looks like from the CURL with production env:

$ curl --request GET \
>   --url 'http://phpslim4.test/v2/pet/findByStatus?status=available' \
>   --header 'accept: application/json' \
>   --header 'x-openapiserver-mock: ping'
{
    "message": "501 Not Implemented"
}

with development env:

$  curl --request GET \
>   --url 'http://phpslim4.test/v2/pet/findByStatus?status=available' \
>   --header 'accept: application/json' \
>   --header 'x-openapiserver-mock: ping'
[{"id":-8738629417578509312,"category":{"id":-4162503862215270400,"name":"Lorem ipsum dol"},"name":"Lorem ipsum dolor sit amet, consectetur adipiscing elit. Lorem i","photoUrls":["Lor"],"tags":[{"id":-3506202845849391104,"name":"Lorem ipsum dolor sit amet, consectetur adipiscing elit. Lorem ipsum dolor sit amet, consectet"}],"status":"pending"}]

with development env, but without required HTTP mock header:

$  curl --request GET \
>   --url 'http://phpslim4.test/v2/pet/findByStatus?status=available' \
>   --header 'accept: application/json'
{
    "message": "501 Not Implemented",
    "exception": [
        {
            "type": "Slim\\Exception\\HttpNotImplementedException",
            "code": 501,
            "message": "How about extending AbstractPetApi by OpenAPIServer\\Api\\PetApi class implementing findPetsByStatus as a GET method?",
            "file": "/Users/ybelenko/Sites/openapi-generator/samples/server/petstore/php-slim4/lib/App/RegisterRoutes.php",
            "line": 861
        }
    ]
}

cc @jebentier @dkarlovi @mandrean @jfastnacht @renepardon

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