I suggest to create interfaces for PHP-client

Description

I need to implement caching for classes. I can use decorators or call cache from each method. But decorators cannot be used with DI because classes don't have an interfaces. What about add interfaces to generation?

Example for java.

Swagger-codegen version

2.2.3

Swagger declaration file content or url

My swagger-magento2-client (generated by swagger).

Command line used for generation

Example of config.json

{
    "composerVendorName": "springimport",
    "composerProjectName": "swagger-magento2-client",
    "variableNamingConvention": "camelCase",
    "invokerPackage": "SpringImport\\Swagger\\Magento2\\Client"
}

java -jar swagger-codegen-cli.jar generate -i http://devdocs.magento.com/swagger/schemas/latest-2.1.schema.json -l php -o /home/swagger-codegen2/tmp/php_api_client -c /home/swagger-codegen2/config.json

Start with the PHP generation path used by the shown swagger-codegen command and config.json, then compare its templates with the Java api.mustache example linked in the issue. Trace how the generated Magento2 PHP client classes are produced. Done means the PHP client generation produces interfaces suitable for dependency injection and caching.