Add OpenAPI Generator

[xepozz]

Reference: https://github.com/OpenAPITools/openapi-generator
https://github.com/openai-php/client

[samdark]

• https://github.com/cebe/php-openapi
• https://github.com/cebe/yii2-openapi

[xepozz]

Good template https://github.com/OpenAPITools/openapi-generator/blob/master/bin/configs/php-nextgen.yaml